You have to use the Pythagorean theorem but there is a little twist here since a square has has equal sides. So:
x^2+x^2=8
2x^2=8
2/2x^2=8/2
x^2=4
√x^2=√4
x=2
The side of the square is 2.
So now the area of a square is:
Side×Side
2×2=4
The area of the square is 4 centimetres
