Ok so I have a PMF of X:

for

(sorry - I don't really know how to write the PMFs on here but that was my best attempt)

we had to work out a which I have as 28, and then which I got to be 0.

However now I have to work out the PMF of Y if

I really don't know how to approach this! I know it's something really simple but I can't think how I would do this!

I know that as the expectation is 0,

but from there I am unsure of how to get the PMF.