Both angles shown ad up to a 90 degree angle. We can see this because on the adjacent side there is a box indicating that it is a right angle. The three angles add up to 180, thus if we subtract 90 degrees due to the angle on the left of the y axis, the angles on the right of the y axis must add up to 90 as well.
Since the angles on the right must add up to 90, we set an equation to solve for x:
