Sole for a. 1 / 3 ( 21 - 3 a ) = 6 - a

a.) all real numbers
b.) - 1
c.) 0
d.) no solution

If the angle bisectors of a pair of opposite angles of a quadrilateral are the opposite sides of a parallelogram formed by the two angle bisectors and two sides of the quadrilateral, is the quadrilateral always a parallelogram? explain your answer.