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
I hope that helped and wasn't to long