The two sides of the triangle are 9 cm and 15 cm. Let the length of the third side is x. According to triangle inequalities, in a triangle sum of the two sides is must be greater than the third side. So, the length of the third side is less than 24 cm.
2x + 40 = 60 (Subtract 40 from both sides) - 40 - 40 -------------------------------- 2x = 20 (Divide both sides) 2x/2 = 20/2 x = 10 (This is your final answer)