This video shows how to compute the area of a rectangle given the length of one of its sides and its diagonal. First start by drawing the picture of the rectangle. Now draw the diagonal and label the known lengths. In the diagram, the diagonal and two sides of the rectangle form a right-angled triangle. The breadth of the rectangle can be computed using the Pythagorean theorem. Finally, compute the area of the rectangle by multiplying its length to its breadth.