If you need more help, please explain what you can do and say where you get stuck.
I guess we'll know more when the OP replies.
Its meaning that X is the smallest and Y is the largest.
so if you rolled the first die and it was a 5 and the second was a 3 the joint pmf would be zero because X can't be larger than Y because its the smallest. Just needed to make sure I did my problem right. Thanks for you post