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 second is given by the equation h=-16t^2+vt+s, where H is in feet. if the object is propelled from a height of 12 feet with an initial velocity of 64 feet per second, its height H is given by the equation h=-16t^2+64t+12. After how many seconds is the height 40 feet?