You have to start with your 2013 tax return. Unfortunately, it is too late to amend your 2013 tax return to get a tax benefit, since it is more than 3 years from the filing deadline. But, you can still get some benefit.
Equipment used in your business is an "asset" (tools for a mechanic, printing presses for a newspaper, instruments for a musician). Assets must be depreciated over their "useful life." Normally, this means taking the wear and tear into account, most assets lose value over time. Machinery wears out, computers become outdated, etc.
The class life for musical instruments is defined as 7 years. That means, roughly speaking you can depreciate, or deduct 1/7 of the value every year over 7 years as a business expense.
There are certain procedures that allow you to take more depreciation -- that is, depreciate the item 50% or even 100% in the first year. If you stop using the item for business before the end of the class life, you have to pay back the excess depreciation. (For example, computers have a class life of 5 years. If you are a graphic artist and buy a new computer and depreciate it all at once, then change careers after 2 years, you have to repay 3 years worth of depreciation.) However, these procedures only apply in the first year, you would have to take them on your 2013 return, and it is too late to amend your 2013 return for a refund.
What you can do now, is file an amended 2013 return to list your instrument as an asset and take regular depreciation over 7 years. Then file amended returns for 2014, 2015 and 2016 to claim those years' worth of depreciation. That carries over to your 2017 return, and for 2 more years after that until the instrument is fully depreciated. The 2013, 2014, 2015 and 2016 amended returns will all show some additional refund due to the added business expenses, but only the 2014, 2015 and 2016 refunds will actually be paid because 2013 is past the allowable date. You would probably also get small refunds from your home state as well if you amend your state returns.
Why you might not want to do this. When you depreciate an item, you reduce it's "cost basis", which you can view as the amount you have invested in the item. If you ever sell it for more than the current cost basis, you have to recapture the depreciation and pay capital gains tax.
For example, if you sell the instrument now for $20,000, you would owe capital gain tax on a $4,000 gain or profit, the difference between the selling price and your cost. But if you take the depreciation deduction and reduce your basis, then you would owe tax on the difference between the selling price and the adjusted basis. If you depreciate the instrument down to zero over 7 years and then sell for $20,000, you would owe 25% depreciation "recapture" on the first $16,000 of the sales price and then 15% capital gains tax on the last $4,000. Also, if you have an insurance loss, you owe income tax if the insurance payment is more than your adjusted cost basis. Again, if you depreciate the instrument and then, for example, an airline destroys it and you receive $16,000 from insurance, that is taxable income because it is more than your basis.
This is rarely a problem with assets like computers or industrial machinery because they rarely gain value over time and almost always lose value. But musical instruments are often different. They can hold their value or even increase in value depending on the type on instrument, maker, and so on.
If you expect this instrument to hold or gain in value, you might be better off not taking the depreciation. (Of course, there is a whole other discussion about present value of money vs future value of money that we could get in to.)
It's up to you, but if you bought the instrument in 2013 you have to go all the way back to 2013 to start the depreciation calculation.
Separately of course, any repairs or maintenance that you paid in 2017 would be a deductible business expense on your 2017 return, without regard for deprecation, and deducting repairs or maintenance doesn't change your cost basis.
