Quadrilateral abcd ? is inscribed in a circle. what is the measure of angle a? enter your answer in the box. m? a= ° a quadrilateral inscribed in a circle. the vertices of quadrilateral lie on the edge of the circle and are labeled as a, b, c, d. the interior angle a is labeled as left parenthesis 4 x plus 5 right parenthesis degrees. the angle c is labeled as left parenthesis x plus 15 right parenthesis degrees.