Answer:

Explanation:
First, we need to find the length of one side. Since it's a square and we have our perimeter of 24, we simply divide that by 4 to get a side length of 6.
When you split a square diagonally, it forms two special 45-45-90 triangles. With such triangles, the leg lengths are x and x, and the hypotenuse is
. Therefore, your answer is
.
I've enclosed a picture to help.