How would I answer this question?

Question

The alligator was the subject of a protection program, the formula P = -10x2 + 475x + 3500 models the alligator population, P, after x years of the protection program, where 0 ≤ x ≤ 12. How long did it take the population to reach 5990 alligators?

Answer 1

P=-10x^2 +475x+3500

5990 = -10X^2 +475X +3500 with t=x = time when pop is 5,990

-10x^2 +475x - 2490 =0

10x^2 -475 +2490 =0

x = 475/20 +/-(1/20)sqr(475^2 -40(2490))= about 19.9374 or -19.9374

= about 20 or -20