a model rocket is shot up in the air and its path is approximated by h=-5t^2+30 where h is the height of the rocket above the ground in meters and t is the elapsed time in seconds, when will the rocket hit the ground?

The rocket will it the ground when it's height is h=0

Set the quadratic equal to 0 and solve.

-5t^{2}+30=0

You will get two solutions. One negative and one positive. Of course, we can not have negative time.

This one is easier because we lack a t term. No quadratic formula or factoring needed.