Step 1: Recall the relation between the areas of the similar triangles.

            THEOREM: The ratio of the areas of two similar triangles is equal to the

            ratio of the squares of their corresponding sides

             \frac{Area \triangle ABC}{Area \triangle DEF} = (\frac{AB}{DE})^2 = (\frac{BC}{EF})^2 = (\frac{CA}{FD})^2   

Step 2: Substitute all the known values in the theorem

            EXAMPLE:   \frac{Area \triangle ABC}{Area \triangle DEF} =(\frac{BC}{EF})^2

                          = \frac{64}{121} =(\frac{BC}{15.4})^2

Step 3: Simplify for the unknown value

          EXAMPLE:   (BC)^2 = \frac{64}{121} *(15.4)^2

                               BC = \sqrt{\frac{64}{121} *(15.4)^2)}

                               BC = \frac{8}{11} * 15.4

                               BC = \frac{123.2}{11}

                                          BC = 11.2 cm