A parallelogram is a quadrilateral in which both pairs of opposite sides are parallel. Parallelograms differ in size of an adjacent sides and angles but opposite angles is equal.
⇒ AB||CD, BC||AD
angle a + angle b = 180 degrees
angle b + angle c = 180 degrees
angle c + angle d = 180 degrees
angle a + angle d = 180 degrees
We have the angles a,b and c.
To solve the problem you would use step D=B-A+C.