The area of a square is the squared side, it means

It means, for A, which has a side of 5, the area is

For B, which side is 12, its area is

For C, the area is

Andre has to paint (solve each power):

Once we add the areas of A and B we realize that the sum is equal to the area of C, it proves the pythagorean theorem that says that the sum of the squared length of the legs equals the squared length of the hypotenuse