AxBxC = {(1,a,y),(2,a,y),(3,a,y),.....}
(AxB)xC = { ((1,a),y),((2,a),y), ((3,a),y)... }
Do not think like that.... you have to understand that
(a,b,c) is not equal to ((a,b),c) because first one is the result of triplet cartesion product, and second one is the result of binary cartesion product.
See I am having international edition of rosen, it has example just above the excercise. You should also check that.. you may see that.