The is the answer on my paper:
The missing side length would be 8
Understanding perpendicular and parallel lines are extremely important, especially in the engineering field when building infrastructures or houses because they need to calculate how they will keep a building stable. For example, bridges we see have a lot of perpendicular and parallel lines, that's because without those the bridge can't hold itself up, it needs support from the metals that are parallel and perpendicular.
hope this helps!!
