Your potato gun has a muzzle velocity of 34 m/s. You want to shoot a potato and have it strike the ground 2.4 seconds later. At what elevation do you have to fire the gun?

h = Vo*t + 0.5g*t^2

h = 0 + 4.9 * 2.4^2 = 28.22 m. = Ht. or
elevation of gun above gnd.