An automobile starts from rest and travels down a straight section of road. The distance $s$ (in feet) of the car from the starting position after $t$ seconds is given by $s(t) = 3t^{3}.$

(a) Find the instantaneous velocity (in feet per second) at $t = 4$ seconds.

Instantaneous velocity at $t = 4$ is ft/s.

(b) Find the instantaneous velocity in (feet per second) at $t = 6$ seconds.

Instantaneous velocity at $t = 6$ is ft/s.