Well that's quite easy. There is a short cut. So 12 divided by 2 is 4. Then 6 divided by 2 is 3. There is a thing called a Pythagorean Triple. And the first one is 3,4 (which is the missing side) and 5. That is the most commonly used set and so are the number's multiples. Therefore the missing leg's length is 4 centimeters.