If r=5, the terms are: a, 5a, 25a, 125a, etc. Let Sn=a+5a+...+(5^(n-1)a= then Sn=a(1+5+25+...+5^(n-1)).
5Sn=5a(1+5+25+...+5^(n-1))=a(5+25+125+...+5^n)
Therefore, 5Sn-Sn=4Sn=a(5^n-1).
When n=6, 4S6=a(5^6-1)=4*2604 and a=10416/(5^6-1)=10416/15624=2/3 (first term).