The functions in part A are correct, we will now use them to solve for part B.
We equate the numbers of the trees in order to find x, which the number of years after which they will be equal.
800(0.95)ˣ = 50(1.15)ˣ
40 = (1.15 / .95)ˣ
Taking log₁₀ on both sides,
log(40) = x*log(1.15/0.95)
x = 19.31 years