For inscribed quadrilaterals, opposite angles are supplementary.
By setting the measures of B plus D equal to 180, we solve for x.
You use this x value to plug into x+15 (measure of angle A).
Opposite angles are supplementary so you subtract the measure of angle A you just calculated from 180 to get the measure of angle C.
\(x+15\\ 73+15=88\\ 180-88=92\)