Thank you to those of you who sent in solutions.

Several of you including Shuaib correctly identified that the lines AB and CD cross at $90$ degrees. Flora observes that the number of squares horizontally and vertically are swapped around when you draw a perpendicular line:

They are the same number of squares although the number of squares has been rotated.

Flora goes on to observe that there is more than one solution each time :

because perpendicular lines can be any distance apart.

Alden and Jimmy from Woodstock Primary correctly point out that :

They do not have to be the same length because they can be any length as long as it's a $90$ degree angle.