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.