$M$ is the midpoint of line segment $P Q,$ and $R$ is the midpoint of $T S .$ If $\overrightarrow{P M}=\overrightarrow{M Q}=\vec{a}$ and $\overrightarrow{T R}=\overrightarrow{R S}=\vec{b},$ as shown, prove that $2 \overrightarrow{R M}=\overrightarrow{T P}+\overrightarrow{S Q}$