return to the ground means h is 0:
0=-16t²+220t
-16t(t-13.75t)=0
t=0 or t=13.75
t=0 is when the rocket is launched, t=13.75 is when the rocket returns to the ground.
b. 756=-16t²+220t
16t²-220t+756=0
use the quadratic formula to solve: t=
so t=7 or 6.75 second
That makes sense because at 6.75 seconds the rocket is at 756 feet going upward, it reaches the highest point, then at 7 seconds it is at the same height of 756 feet coming down.