Answer :
Find the slope for the two lines and state that they are equal (condition for parallel lines)
Slope of line AB
[y2 - y1] / [x2 - x1]
Slope of line CD
[y4 - y3] / [x4 - x3]
Condition
[y2 - y1] / [x2 - x1] = [y4 - y3] / [x4 - x3]
Slope of line AB
[y2 - y1] / [x2 - x1]
Slope of line CD
[y4 - y3] / [x4 - x3]
Condition
[y2 - y1] / [x2 - x1] = [y4 - y3] / [x4 - x3]