The basic idea is this definition of logarithms.
Given α>0, α=1, and β>0,logα(β)=γ⟺β=αγ.
(From that definition, topsquark’s hint follows directly, but I actually think it is easier to work from the definition itself.)
The Cat has given you one method, but I find it a bit easier to go
−93=20log10(d)+20log10(868000)+20log10(2997929484π)−3−6⟹−84=20log10+20log10(2.997,929,480∗1083.472∗106∗π)⟹log10(d)+log10(2.997,929,4803.472∗π∗10−2)=−2084⟹log10(d)−2log10(2.997,929,4803.472∗π)=−4.2.
Now can you finish it up?