Answer:
t = ±10
Explanation:
after using the FOIL method you have the following:
490 + 49t - 49t - 4.9t²
middle two terms zero out
h(t)= -4.9t² + 490
if you multiply by -10 you can get perfect squares
h(t) = 49t² - 4900 which has two factors: (7t + 70)(7t - 70)
now set each factor equal to zero and solve:
7t + 70 = 0
7t = -70
t = -10
7t - 70 = 0
7t = 70
t = 10