Hi there!
When two lines intersect, we know that the ones on the opposite sides are equal to one another. Thus, we know that the angle opposite of the one marked with 90 degrees (on the other side of the intersection) is also 90 degrees.
We know that a full turn is 360 degrees. We can see that the two 90 degrees make up part of the turn, and the other pair of angles, both of which are equal to x (as per the reasoning above), make up the rest. We know that the two 90 degrees, added together, is equal to 180 degrees. This means, after subtracting 180 from 360, that 180 degrees are remaining that aren't the 2 90 degrees, and are make up of two angles both measuring x degrees.
As we know that they both measure x, then we know that we say 2x = 180, which gives us x = 90.
Hope this helps!
