Let x be the length of either of the two congruent sides. Then the third side is (2x-8), and you have this equation for the perimeter x + x + (2x-8) = 40. From the equation 4x - 8 = 40 4x = 40 + 8 = 48 x = 48/4 = 12. ANSWER. The sides are 12 units, 12 units and 2*12-4 = 20 units long.