A parallelogram, a rectangle and a triangular region stand on the same base and between the same parallels. If their areas are $P$, $R$ and $T$ respectively, then

Let the common base be $b$ and the distance between the same parallel lines (height) be $h$.
The area of a parallelogram is given by:

$$P=b\times h$$

A rectangle is also a parallelogram, so its area is:

$$R=b\times h$$

The area of a triangle is given by:

$$T=\frac{1}{2}\times b\times h$$

From these equations, we get:

$$P=R=2T$$