When a line bisects another in a certain point it creates two segments, these must be equal in length. So the sum of their lengths is equal to the length of the whole line. In this case the line QR was divided in PQ and PR, so we have:
We want to find the length of PR, so we need to isolate it on the left side:
Applying the expressions for QR and PQ we have:
The segment PR measures "6x - 12".