(a) Compare your quadratic for h to the general quadratic ax² +bx +c. Perhaps you can see that ...
a = -16
b = 128
You use these numbers in the given formula to find the time when the ball is highest.
t = -b/(2a) = -128/(2(-16)) = 4 . . . . . . the time at which the ball is highest
(b) Evaluate the quadratic to find the height at t=4.
h = -16(4)² +128(4) +21
h = -256 +512 +21
h = 277
The maximum height of the ball is 277 ft.