a. 13x can mean the number of bags of marbles times the number of marbles in each bag (the total number of marbles in 13 bags)
b. 5 can represent the initial numbers of marbles one has. For example, even before purchasing 13 bags of marbles you already had 5 marbles from before (they are like 5 extra marbles)
c. x can stand for the number of marbles in each bag