You are trying to solve for n. To do so, you need to know what logorithms are. I'm hoping you do:
100(1.20)^n = 300
1.20^n = 3
log(1.20^n) = log(3)
n*log(1.20) = log(3)
n = log(3)/log(1.20)
Plug this into a calculator to find what it equals.
By the way, if you don't understand any (or all) of the steps that I just did, say so and I can explain.