What is the formula for present value.

For compounded interest:

FV = PV (1 + I)^n

I prefer to use that equation and just use algebra to solve for PV.

An actual equation for PV is basically just twisting it around to:
FV/
[(1 + I)^n]

Either way, you have to do the parenthesis and exponent first, and then divide FV by your answer.

Where I = interest rate per compounding period, and n = total number of compounding periods. Different books can use different letters there.