The displacement of a particle moving along an $x$ axis is given by $x=18 t+5.0 t^{2}$, where $x$ is in meters and $t$ is in seconds. Calculate (a) the instantaneous velocity at $t=2.0 \mathrm{~s}$ and (b) the average velocity between $t=2.0 \mathrm{~s}$ and $t=3.0 \mathrm{~s}$.