Do not just send the IRS any money without first contacting them about this notice. I would get a local CPA involved unless you feel like you are comfortable in dealing with the IRS on a personal level. The IRS is actually quite helpful and empathetic to taxpayers. I'd go back to your 2015 income tax return and see if the Social Security income was included or not. If it was not included on the tax return, then there is little you can do except pay the taxes that would have been paid had the income been included on the 2015 tax return to begin with. The IRS is an extremely patient and understanding organization. That being said, any penalty they assess can be abated (reduced or eliminated entirely) but it will have to be shown to the IRS that you did not intentionally leave this income off your tax return. There is an IRS webpage that lists all the reasons why they will or will not abate the penalty, but it is lengthy and boring (this is why I suggested getting a local CPA).

The website is: https://www.irs.gov/irm/

In particular, you wanna look in Part 20 Penalty and Interest (20.1.2.1.4.1 (07-18-2016) Penalty Abatements and Re-assessments)

If you find that the Social Security income was included and that the IRS is incorrect in their assessment, you must communicate this to the IRS. Provide them any documents that help to prove your position. The key here is to contact and communicate with the IRS as soon as possible to keep this from getting any messier.