The ball height is 0, when it hits the ground. So

So from the equation, -16t = 0 or (t - 6) =0.
Evaluate the value of t for the ball hit the ground.

OR

The ball height is 0 at t = 0 sec and at t = 6 sec.
The t = 0 sec represent the initial condition and t = 6 sec represents the time after which ball hit the ground again.
So answer is t = 6 seconds.
PART B.
Differentiate the equation and equate to 0 for maximum height.

For maximum height,

So part B answer is 3 sec
Answer:
Part A: 6 seconds
Part B: 3 seconds