What are all values of T for which the balls height is 11 feet

Question

Find all values of T for which the balls height is 11 feet

A ball is their own from initial height of 5 feet with an initial upward velocity of 21 feet/S. The balls height h (in feet) after T seconds is given by the following.

h=5+21t-16t²

Answer 1

11 = 5+21t -16t^2

16t^2 -21 + 6 = 0

t =21/32 + or - (1/32)sqr(441-384)

t = 21/32 + or - (1/32)sqr(57)

t = (21+ or -sqr57)/32

T = 0.8922 seconds and 0.4203 seconds are when the ball is 11 feet high