[SOLVED] population growth differential equation problem

Consider a population p of a field mouse that grows according to the following differential equation: dp/dt = 0.5p - 450

* find the time at which the population becomes extinct if p(0) = 850

* find the time of extinction if p(0) = p(sub zero), 0 < p(sub zero) < 900

* find the initial population p(sub zero) if the population is to become extinct in 1 year

im not sure how to find the time given the equation is in terms of population. can anyone help, please...?