What is the length of the diagonal of a square with area 40 cm^2
The area of a square is:
$\displaystyle A = s^2$
Using this you can find the length of the a side of a square. To find the length of the diagonal, think of it as the hypotenuse $\displaystyle c$ of a triangle with legs $\displaystyle s$. Then, using the Pythagorean Theorem:
$\displaystyle s^2 + s^2 = c^2$