(20points) While practicing a tightrope walk at a height of 20 feet, a circus performer slips and falls into a safety net 15 feet below. The function h(t) = -16t^2+20, where t represents time measured in seconds, gives the performer's height above the ground (in feet) as he falls. Write and solve an equation to find the elapsed time until the performer lands in the net. You can solve using a graphing calculator.