Well its been a while since i've done any physics problems and im having a problem with a real simple problem. The problem is: You know that the height (in m) of a projectile is given by the formula h = - g * t^2 / 2 + vi * t where t is the time from launch, g is the acceleration due to gravity (9.8 m/sec2) and vi is the initial vertical velocity component (in m/sec). Calculate the time at which a projectile will reach a certain height. vi = 20 desired height = 10 So far i have this done, but i always get a negative number and the answers should be 0.58 sec and 3.50 sec. 10 = -9.8t^2/2 + 20t 10(2 + 20t) = -9.8t^2 20 + 200t = -9.8t^2 9.8t^2 + 200t + 20 = 0 (-200 + sqrt(200^2 - 4(9.8*20)) / 2(9.8) -200 + sqrt(40000 - 784)) / 19.6 (-200 + 198.0303)/19.6 Im sure its just a stupid math mistake, but i cant find it. Also where we even given the correct equation?