Let's draw a diagram to represent the given problem.
If R is midpoint then PR and RQ are equal by definition of midpoint. So, we can express the following equation.

Then, we solve for x. First, we subtract 5x on each side.

Now, we subtract 6 on each side.

At last, we divide the equation by -2.

The solution for x is 4.
We use this value to find PR and RQ.

Then, we find PQ.

Therefore, PR and RQ are equal to 18 units, and PQ is equal to 36 units.