By the fundamental theorem of calculus, if
p'(t) = 90t + 500/(1 + t)
and p(0) = 2000, then
p(t) = p(0) + ∫₀ᵗ p'(u) du
⇒ p(t) = 2000 + ∫₀ᵗ (90u + 500/(1 + u)) du
⇒ p(t) = 2000 + (45u² + 500 ln|1 + u|)₀ᵗ
⇒ p(t) = 2000 + 45t² + 500 ln|1 + t|
Then after t = 3 hours, the population of bacteria would be
p(3) = 2000 + 45•3² + 500 ln(1 + 3) ≈ 3098