Anton bakes 36 cookies. He gives cookies to each of his 3 friends.

Let c represent the number of cookies he gives each friend.

Which expression shows how many cookies Anton has left?

The answer is a because if he has 36 cookies you start with that number. Then you don't know how many cookies he gave to his three friends so you have c as the variable that is multiplied by three. These two sets connect (36-cookies to begin with) minus (because he is giving away so he doesn't have them) and (3c-becuase 3 friends times the amount of cookies he gives each[unknown value=c])

So put together it's

36-3c