Okay so Marty's age is m. That means we have NO IDEA how old he is. He could be 93 years old or 15 years old. We have no clue. Since he could be any number we will symbolize that number with m. Now it says that Amy is four years younger, this means we have to subtract 4 from any age that Marty is...
this will give us: m-4
and that is your answer