$\begin{align}  & 3s\le t\le 6s \\  & {{v}_{av}}=\frac{{{v}_{1}}+{{v}_{2}}}{2}\xrightarrow{{{v}_{1}}=a{{t}_{1}},{{v}_{2}}=a{{t}_{2}}}{{v}_{av}}=\frac{a({{t}_{1}}+{{t}_{2}})}{2} \\  & \xrightarrow[a=4\frac{m}{{{s}^{2}}}]{{{t}_{1}}=3s,{{t}_{2}}=6s}{{v}_{av}}=\frac{4\times (3+6)}{2}=18\frac{m}{s} \\ \end{align}$