Aball is thrown into the air. the height h, in feet, of the ball can be modeled by the equation h= -16t^2 + 20t + 6, where t is the time, in seconds, the ball is in the air. when will the ball hit the ground?

Hello : let h =f(t)
the ball hit the ground if : f(t) =0
solve :  -16t² + 20t + 6=0
Δ = b²-4ac  ... a = -16    b= 20     c =6
Δ =(20)²-4(-16)(6) = 784 = 28²
t1 = (-20-28)/-32 = 48/32 =3/2
t2 =(-20+28) /-32 < 0 refused
conclusion : t=3/2