We have the following drawing
Note that angle PBR and angle RBM are complementary. That is, their measures add up to 90°. Let x be the measure of angle PBR. So we have the following equation
Then, by subtracting 41 on both sides we get
Then angle PBR has a measure of 49°.
Note that angle PBR is the sum of c and 53. So we have the following equatio
By subtracting 53 on both sides, we get
Thus, the value of C is -4.