In a quadrilateral $PQRS,\vec{P}Q=\vec{a},\vec{Q}R,\vec{b},\vec{S}P=\vec{a}-\vec{b},M$ is the midpoint of $\vec{Q}RandX$ is a point on $SM$ such that $SX=\frac{4}{5}SM\dot{}$ Prove that $P,XandR$ are collinear.

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