They want you to solve for r in terms of the other pronumerals. I suggest that you first attempt to make ln(r-1) the subject.

By the way, your expression is ambiguous. Is it meant to be V=B-q/[k In(r-1)] or V=[B-q/k] In(r-1) ....?? Or V=(B-q)/[k In(r-1)] or V=B-(q/[k In(r-1)]) etc. etc. Please use bracketes, or take the couple of minutes to learn how to use latex (click on the appropriate link in my signature).