logbx+logby-logbz
=logb(x*y)/z if b is the base of log.
There are two properties that you can apply to get the correct answer.
logb(x*y)=logbx+logby if b is the base of log.
logb(x/y)=logbx-logby if b is the base of log, so the correct answer for logbx+logby-logbz=logb(x*y)/z. Hope it help!