Question 1177056
slope of line = (y2-y1)/(x2-x1) where )x1,y1) and (x2,y2) are the two points on the line

Slope of AB = (2b-0)/(2a-0)= 2b/2a=b/a

slope of CD = b/a ( parallel to AB)

slope of line = (y2-y1)/(x2-x1) where )x1,y1) and (x2,y2) are the two points on the line

slope of CD = ((2b+2d)-2d) /( (2a+2c)-2c) = b/a 


Therefore C = (2a + 2c, 2b + 2d) completes the
parallelogram