Answer: 12.
To solve this problem, let's first solve for x. Thi is easiest done by figuring what QR is in terms of x using two equations, both from different lines.
In the first line:
QR = 15 - 4x.
In the third line:
QR = 13x - 1 - x = 12x - 1.
Now, we have to set these equations equal to each other.
15 - 4x = 12x - 1
15 = 16x - 1
16x = 16
x = 1
Next, we take line three, 13x - 1, and substitute x as 1. The answer is 12.