Hello,
When you have an polygon inscribed in a circle, the opposite angles are supplementary.
Therefore, we can write and solve the following equation.
2x + 3 + 4x + 3 = 180
6x = 174
x = 29
If you substitution 29 into the expression for C, you will find that C = 59.
2(29) + 1 = 59
Good luck,
MrEQ