An automobile starts from rest and travels down a straight section of road. The distance $y$ (in feet) of the car from the starting position after $x$ seconds is given by $$y = f(x) = 3 x^2.$$

(a) Find the instantaneous velocity at $x = 4$ seconds (include units).
Instantaneous velocity =

(b) Find the instantaneous velocity at $x = 6$ seconds (include units).
Instantaneous velocity =