Mid-point area

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

Letting RM be the base of the triangle:

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