## Precalculus (6th Edition) Blitzer

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