Hello Twan,
I assume that the function p(t) that you gave represents the population of the insect in question as a function of time t (not the "size" of an individual insect.) If so, we can solve the problem as follows. Simply set p(t) = 2000, and solve for t.
p(t) = 2000
500e.03t = 2000.
Dividing each side by 500 gives
e.03t = 4
Now take the natural logarithm of each side.
Ln(e.03t) = Ln(4)
Since Ln(eA) = A for all A, the above equation becomes
.03t = Ln(4)
t = [Ln(4)]/.03
Using a calculator to get the value of Ln(4), we obtain the final answer.
t = 46.21 days (rounded to the second decimal place.)
Hope that helps! Let me know if you need any further explanation.
William