A particle moves according to a law of motion $s=f(t)=t^3-12t^2+36t+ 3 , \; t\geq 0$, where $t$ is measured in seconds and $s$ in meters.
(a) Find the acceleration at time $t$.
(b) When is the particle speeding up?
(c) When is the particle slowing down?

(a) $a(t) =$
(b) $< t <$ and $t>$
(c) $0 \leq t <$ and $< t <$