What is the tax penalty from withdrawing from an investment account (wealthfront) for a downpayment on first home? I make around 30,000 per year and have 48,000 in investments.

There’s no penalty for withdrawal from a non-retirement account unless you are cashing out a CD prematurely, for example. If you are liquidating an asset that has a capital gain, you would of course be taxed on that income (15%). 

and depending on your income level, that capital gains tax may be less than 15%. (and don't forgest state income tax!)

capital gains is the difference between your 'cost basis' and the proceeds from the sale of the investments. Your broker should have good records on these numbers.