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. 
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