LOL, this one I spent 10 minutes on. It is kind of hard, but here's the logic:
Use pythagorean's theorem and you find out that the third side has to be:
c = sqrt(a^2+b^2) = sqrt(13^2+5^2) = 13.928
The side drawn ISN'T actually 13.928. Why? It's an illusion. If you would zoom in on the picture, it would actually look like this:
When your rearrange the pieces, it now looks something like this:
The piece that was put out (see below) is now removed by the one empty square, so that the area will still remain equal.
= empty square
I admit this problem was pretty tricky