The diagonals would bisect each other if it were a parallelogram. That means "y2" would equal "y + 30" and "x" would equal "3x - 40".
If: y2 = y + 30 then:
y2 - y - 30 = 0
(y - 6)(y + 5) = 0
So either y - 6 = 0 (meaning y = 6) or y + 5 = 0 (meaning y = -5). If y = 6, then the diagonal would be 72 units long (each segment would be 36). If y = -5, then the diagonal would be 50 units long (each segment would be 25).
If: x = 3x - 40, then:
2x = 40 or x = 20.
If x = 20, then the diagonal would be 40 units long (each segment would be 20)