Explanation:
you have 18 choices for the birthday cards.
for each of these 18 choices you have 6 choices to get well cards. that means there are 18×6 choices just for these 2 card types alone.
for each of these 18×6 choices you have then 9 choices of graduation cards. that is in total 18×6×9 choices.
for each of these 18×6×9 choices you have then 12 choices of anniversary cards. that is in total
18×6×9×12 choices = 11,664 choices or ways the cards can be selected.