Let's go:
The circumcenter is the intersection of the perpendicular bisectors on the sides of a triangle. An important property of the circumcenter is the fact that it is equidistant from its vertices. PC = RC = QC:
3x + 7 = 51 - x
3x + x = 51 - 7
4x = 44
x = 11
I hope I helped you.
