You can use c^2 = a^2 + b^2 to solve for the length of the other leg. Just substitute the values to the variable then transpose to get the value of the missing variable.
Given: a = 13; c = 21 Required: b - other leg Solution: c^2 = a^2 + b^2 21^2 = 13^2 + b^2 441 = 169 + b^2 b^2 = 272 <span>√b = 16.49 b = 16.49 cm</span>