I've been struggling with this particular distribution. I'm told that and have a trinomial distribution with parameters , and

I know that the joint pmf is

I'm asked to find various things, such as

and

I don't want anyone to solveallof these.

However, if someone can show me how to find the expected value of either x or y, I would be able to figure out the rest.

I have an inkling that , but how would I find the marginal of ???

I'd appreciate any input!!

--Chris