Using Pythagorean Theorem
a^2 + b^2 = c^2 where 'a' and 'b' are the short sides (legs) and 'c' is the hypotenuse (the longest side)
Example : right triangle with legs of 4 and 'x' and hypotenuse 5
4^2 + x^2 = 5^2
16 + x^2 = 25
x^2 = 9
x= 3 = this 'missing' leg size.....