Barry, a tennis pro, bought y cases of tennis balls. Each case contains x cans of tennis balls. Each can contains 3 balls. What expression would represent the total of tennis balls?

Y x X x 3

since Y is how many packages there is, and X is how many cans there are in each package, Y x X would equal the total amount of cans. Multiplying the total number of cans by 3 (since there are 3 balls in each one) would get you the total amount of tennis balls there are.