Approximately how much will i be taxed if i wont 35000) on Stake.us playing sweepstake?

You will be taxed at your marginal tax rate. 

We cannot tell you how much the tax will be. It depends on your other income and many other factors.

You could prepare a dummy tax return will all of your other income, deductions, and credits, then add the gambling income and see how much the tax increases. If you do that, do not use the same TurboTax Online account that you use for your actual tax return. You do not want to modify your actual tax return. If you create a separate account for the dummy tax return, be sure to keep track of which account you are using for your actual tax return and which account is the dummy.