another application problem (population)

The US prison population followed the curve

N(t) = 0.028234t^3 – 1.0922t^2 + 13.029t + 146.88

(0 __<__ t __<__ 39)

in the years 1950 – 1989. Here t is the number of years since 1950, and N is the number of prisoners in thousands. When to the nearest year was the prison population decreasing most rapidly? When was it increasing most rapidly?

The took the derivative N(t) and set it equal to zero to find stationary points. I ended up getting answers the answers 16 and 9 but I know those are the wrong answers