We can model the situation as:
Since Q is the midpoint of PR, QR and PQ have the same length, so PQ is also equal to 43.
Now, we can formulate the following equation:
PR = PQ + QR
So, replacing PR by 9x-31, PQ by 43 and QR by 43, we get:
9x - 31 = 43 + 43
9x - 31 = 86
Solving for x:
9x - 31 + 31 = 86 + 31
9x = 117
9x/9 = 117/9
x = 13
Answer: x = 13