Hey there!
Notice in the image, I is in between Q and C and QC is made up of QI and IC.
Knowing this, you can conclude that QI+IC=QC
Since you know that QI=3x-13 and IC=2x-12, you can add these two expressions together to get QC:
(3x-13)+(2x-12)
5x-25
Therefore, QC is 5x-25.