Kinematics of motion in a straight line — practice question

A particle $X$ travels along a straight line. Its displacement from $O$ at time $t$ after leaving $O$ is $s\,\text{m}$, where $s = 0.3t^2 + 0.6t$ for $0 \le t \le 4$. For $t > 4$, the acceleration of $X$ at time $t$ after leaving $O$ is $a\,\text{m s}^{-2}$, where $a = 0.3t^{\frac{1}{2}}$. The velocity of $X$ does not change at $t = 4$. The velocity of $X$ at $t = T$ is $14.2\,\text{m s}^{-1}$.