How many different rectangle s are there on a chess board?How many of these have area =3×area of small square

A rectangle is formed by two horizontal lines and 2 vertical lines. How many ways can you choose two vertical lines? Well there are 9 total. So 9c2. Order doesn't matter, which is why we use combinations . This equals 36.

How many ways can you choose two horizontal lines. There are 9c2. Again order doesn't matter, so we use combinations. It equals 36 again.

36 * 36 = 1296