first the ball dropped at time = 0 seconds so by substitute t= 0 in the equation you get that the hight = 150 ft h(0)=-16.1(0)^2+150 so , h=150 ft second when the ball just touches the ground the hight will be 0 , so by substitute by h=0 we get 0=-16.1t^2+150 t^2 = -150/-16.1 t= 3.052 second
