This is 5 points dude, not 100. (jk I'll help!)
To find tax, you would multiply your total amount by your tax percentage. You didn't provide us with what the tax percentage was, so I'll give you what you should do, which is:
D=710+(710*t)
Where t is your tax percentage and D is your final amount in dollars.
If you can provide us with the tax percentage, I would be happy to help you do that as well.
:)