Hi
This formula gives you the position of the last "a" in A1:
=FIND("|",SUBSTITUTE(A1,"a","|",LEN(A1)-LEN(SUBSTITUTE(A1,"a",""))))
Remark: I used the character "|" as auxilliary, assuming there's no "|" in A1. If there is, choose another character not in A1, like for ex. "#".