Solving a word problem using a quadratic equation with irrational

Question

A model rocket is launched with an initial upward velocity of 30/ms The rocket's height h (in meters) after t seconds is given by the following.

Find all values of for which the rocket's height is meters.

Answer 1

h(t) = -4.9t^2 + 30t = m = height of rocket measured in meters

solve for t = time rocket is at m height

4.9t^2 -30t + m= 0

t = 30/9.8 + or - (1/9.8)sqr(900-4(4.9)m)

plug in the height m and solve for t to get 2 different times when the rocket is at that height