The area of a trapezoid is
where B and b are the two bases, and h is the height. The two bases have to be parallel, and the height must be perpendicular to both bases.
Even though we are used to have horizontal bases and vertical height, it's not mandatory, since a simple rotation doesn't affect the dimensions or the area of the trapezoid.
So, in this case, the two vertical segments are actually the bases of the trapezoid, and the horizontal segment is the height. So, the formula for the area yields