f(x) represents the population after x amount of years. So, you are given f(x) = 15 000 000 and 20 000 000 and must solve for x (i.e. the amount of years after 2000). So:

Take the logarithms of both sides and solve for x. Come back if you still have trouble!

Edited: Misread 7 400 000 for 74 000 000