12 cm The solution to this problem requires the Pythagorean theorem which is a^2 + b^2 = c^2 where a,b = legs of the right triangle c = hypotenuse of right triangle Let's substitute the known values into the equation and solve a^2 + b^2 = c^2 5^2 + b^2 = 13^2 25 + b^2 = 169 b^2 = 144 b = 12 So the length of the 2nd leg is 12 cm.