At time t=0 the object will be at its highest point....the point where it started its fall
substitute t= 0 in to the equation
h(t) = - 16(0)^2 + 900 = 900 ft
The object will hit the ground when the equation is EQUAL to 0....
0 = -16t^2 + 900
900 = 16t^2
900/16 = 56.25= t^2 so t=sqrt(56.25) = 7.5 seconds to hit the ground.