How does this difference show that whales and animals with four legs are probably descended from a common ancestor?

The question is unfortunately incomplete. The difference referred to here is that related to what s cientists discovered between today's whales and the fossils of whales' ancient ancestors. The scientists discovered that the ancestors of whales had four legs and could walk on land. This commonality in body form between land animals and the ancestors of whales indicates a common ancestor.

The common ancestry described above may be because of the homologous structures. Homologous structures are organs or bones that appear in different animals, underlining anatomical commonalities demonstrating descent from a common ancestor. Examples of homologous structures include the arm of a human, the wing of a bird or a bat, the leg of a dog and the flipper of a dolphin or whale among others.