if an object is propelled upward from a height of s feet at an initial velocity of v feet per second, then its height h after t seconds is given by the equation h= -16t^2 + vt +s, Where h is in feet. if the object is propelled from a height of 4 feet with an initial velocity of 64 feet per second, its height h is given by the equation h= -16t^2 + 64t + 4.

after how many seconds is the height 64 feet?