$\begin{array}{l}
\overrightarrow {MA} + 2\overrightarrow {MB} + \overrightarrow {MC} = \left( {\overrightarrow {MA} + \overrightarrow {MC} } \right) + 2\overrightarrow {MB} \\
= 2\overrightarrow {MN} + 2\overrightarrow {MB} = 2\left( {\overrightarrow {MN} + \overrightarrow {MB} } \right)\\
= 2.2\overrightarrow {MI} = 4\overrightarrow {MI}
\end{array}$