Sharon kicks a ball from the ground into the air with an upward velocity of 64 feet per second. The function h = -16t2 + 64t models the height h, in feet, of the ball at time t, in seconds. When will the ball reach the ground again? 1 second after the ball is thrown

2 seconds after the ball is thrown

3 seconds after the ball is thrown

4 seconds after the ball is thrown

i believe that the answer is 4 seconds but i dont understand how to work it out