If Log_y6=x, Log_y3.5=z and 6a=3.5 then a=??

If Log_y6=x, Log_y3.5=z and 6a=3.5 then a=??

(a) xz
(b) z/x
(c) z+x
(d) z-x

I searched on internet about correct answer and it was (b) z/x, but I don't know why (b) is correct. I think that all the answers listed are incorrect.