100 points with brainleist

How do you use the elimination method for this question? explain, because i really want to understand!

For each proportion find the unknown number x 1/5/8=×/15

The height of a flare fired from the deck of a ship in distress can be modeled by h(t)= -2(8t^2-52t-28), where h is the height of the flare above water and t is the time in seconds. a. find the time it takes the flare to hit the water.