If two sides are parallel, you multiply the average of the lengths of the parallel sides by the (perpendicular) distance between them. Otherwise, it depends on the particulars of the shape.
How do you determine the angle degrees of geometric shapes? I know that a triangle has 180 degrees, what about quadrilaterals or polygons?
Find the area of quadrilateral whose sides are 9m, 40m, 28m and 15m