Match the corresponding sides of the figures. This is easy if the figures are drawn the same exact way. However, figures are often flipped in a different direction. In this case, you can trace one figure onto a separate sheet of paper and turn or flip it as needed until it matches the first figure.
Find the missing side on the figure and compare it to the corresponding side on the congruent figure.
Write down the measurement from the reference figure as the measurement for the missing side of the other figure. For instance, consider a triangle with its sides labeled as 2 inches, 3 inches and 5 inches. If you have a second triangle that is congruent, its longest side will be 5 inches. The next longest side will be 3 inches, and the shortest side will be 2 inches.