The equation represents height above the ground. we want to know time it takes to hit the ground, so make the equation (height) equal to zero and solve for t.
[tex] 0 = - 16{t}^{2} + 120 \\ \\ - 120 = - 16 {t}^{2} \\ \\ 7.5 = {t}^{2} \\ \\ t = 2.7 \: seconds[/tex]
