To determine the area of a region that is composed of rectangles and triangles, you should just calculate for the individual area of each of the rectangles and the triangles. Then, after determining the areas, you add them all up. The sum would represent the total area of the region. The area is the number of square units in a shape. It is one way of describing the size of a particular surface. For a rectangle, it is calculated from the product of its length and its width. For a triangle, it obtained from one half the product of its base and the height.