If a Ball Is Thrown into the Air with a Velocity

Short Answer

If a ball is thrown into the air with a velocity of $58 \mathrm { ft } / \mathrm { s }$ , its height (in feet) after $t$ seconds is given by
$$H = 58 t - 9 t ^ { 2 } \text {. }$$
Find the velocity when $t = 9$ .