The Pythagorean theorem can be shown with the formula a²+b²=c². This means that if you square the two sides of a right triangle and add them together, they will equal the hypotenuse squared.
In this case, the smallest side of the triangle has 36 squares, which is the same as 6². The larger side of the triangle has 64 squares, or 8². If you do 36+64, you get 100, the same thing as 10². Put all that together and you should get 6²+8²=10², following the formula a²+b²=c².