Recall that the area of a parallelogram is base x height.
If you divide a parallelogram into two congruent triangles, the area of each triangle is ½ x base x height.
So the Area of a Triangle: A = ½ bh
In order to use the lengths of the legs of the triangle as the base and height, the legs must meet at a 90o angle.
Area of a Trapezoid
A trapezoid can be divided into a rectangle and two triangles. The area of a trapezoid is the sum of the areas of the rectangle and the triangles.
So the Area of a Trapezoid is
You can follow these steps at this site. Go there to read these steps and see the images.
Area of Irregular Polygons
You can find the area of irregular polygons by decomposing the polygon into rectangles, triangles and other shapes.
Find the area of each shape.
This is 27, 24, 18, 30.