$\begin{align}
  & \left. \begin{matrix}
   {{({{{\vec{v}}}_{av}})}_{1}}=\frac{{{{\vec{d}}}_{1}}}{\Delta {{t}_{1}}}  \\
   {{({{{\vec{v}}}_{av}})}_{2}}=\frac{{{{\vec{d}}}_{2}}}{\Delta {{t}_{2}}}  \\
\end{matrix} \right\}\xrightarrow[{{({{{\vec{v}}}_{av}})}_{2}}=18\vec{i},\Delta {{t}_{2}}=4s]{{{({{{\vec{v}}}_{av}})}_{1}}=6\vec{i},\Delta {{t}_{1}}=2s}{{{\vec{d}}}_{1}}=-12\vec{i},{{{\vec{d}}}_{2}}=75\vec{i} \\
 & {{({{{\vec{v}}}_{av}})}_{3}}=\frac{{{{\vec{d}}}_{1}}+{{{\vec{d}}}_{2}}}{{{t}_{3}}-{{t}_{1}}}=\frac{(-12\vec{i})+(72\vec{i})}{8-2}\Rightarrow {{({{{\vec{v}}}_{av}})}_{3}}=10\vec{i} \\
\end{align}$