I don't get the pythagorean theorem

How does it work ?
How do you find the value of one of the legs of a triangle?
I know that The legs squared (a squared + b squared = C squared, where C is the hypotenuse) but how do you find the value of a leg if you have the value of a hypotenuse and a leg ?

...that's what I just said, but you just reworded it. :/

That's redundant, isn't it? :/