#### Answer

$-24$ feet /second in the downward direction

#### Work Step by Step

$ s(t)=-16t^2+72t $
Thus, we have $ v(t)=sā²(t)=-16 \dfrac{dt^2}{dt}+72 \times \dfrac{dt}{dt}$
So, $ v(t)=-32t +72$
Set $ t=3$
Then $ v(3)=-32(3) +72=-24$ feet /second in the downward direction