the complete question in the attached figure
we know that
∠B+∠D=180°------------> supplementary angles
Because an polygon inscribed in a circle, the opposite angles are supplementary.
so
(2x+3)+(4x+3)=180-----------> 6x+6=180--------> x=(180-6)/6-----> x=29°
m∠C=2x+1---------> 2*29+1-------> 59°
the answer is
m∠=59°
