Answer
See below
Work Step by Step
Given: $h_0=9\\v_0=-40\\h=0$
The formula for the height: $h=h_0+v_0t-16t^2$
Substitute: $-16t^2-40t-9=h$
Setting $h=0$, we have: $-16t^2-40t-9=0$
The solution is: $t=\frac{40 \pm \sqrt (-40)^2-4(-16)(9)}{2(-16)}\\=\frac{40 \pm \sqrt 2176}{-32}\\=\frac{-40 \pm \sqrt 2176}{-32}$
Hence, $t \approx 0.21 \lor t\approx-2.71$
Since time cannot be negative, we will choose $t=0.21$ sec
