Using relatedtable insteard of related function

Hello,

The formula below is failing because I am trying to use the RELATED function on the many side of a 1:many relationship. I've read that it should work using the function "RELATEDTABLE" but I can't seem to get my head around it - please can you tell me how to edit the highlighted code below? 

Thank you,

CM

(p.s. I know there are better ways of finding the "count of unique customers for prior fiscal year" but I'm just practicing how to use the functions)