s(t) = 100 - ½gt2 - 5t when t-0, s(0) = 100. This will decrease with time. Sign of velocity term (5t) is negative because time will cause height to decrease. The gravity term, where g = 9.81 m/sec, is also negative because gravity will pull downward also.
Solve the equation for t when s(t) = 0. USE WHATEVER g VALUE IS TYPICAL IN YOUR CLASS; some books use 9.8, others may use g=10 for the sake of convenience.