Please help me. I’m confused

Question

The Height of an object drop from the roof of 9 story building is given by H = -8t² + 82 where H is the height of the object from the ground in feet and t is the number of seconds elapsed. Give the time interval when the object is at least 10 ft from the ground. 

Answer 1

-8t^2 + 82 = 10

-8t^2 = -72

t^2 = 9

t=3

height is 10 when t=3, so H(3) = 10

t = [0,3]