$M$ is the midpoint of the side of the rectangle, so $RM$ has length $2$ units.

\begin{eqnarray} \text{Area of Triangle } PMR & = & \frac{1}{2} \times \text{base} \times \text{height} \\ & = & \frac{1}{2} \times 2 \; \text{units} \times 3 \; \text{units} \\ &=& 3 \; \text{square units}\end{eqnarray}

*This problem is taken from the UKMT Mathematical Challenges.*