Question

If I have a parallelogram ABCD , i have points A, B, and C. I need to get D
which rule I should use ?
(D=A-B+C) or
(D=B-A+C) ?

Answer 1

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.