\(2\log{(a -2b)} = \log{a} + \log{b}\\ \log((a-2b)^2)=\log(ab)\\ (a-2b)^2=ab\\ a^2-4ab+4b^2=ab\\ a^2-5ab+4b^2=0\\ \)
You can factor this polynomial like this:
\((a-4b)(a-b)=0\)
So the two solutions for a/b are:
\(a-4b=0\\a=4b\\ \boxed{\frac{a}{b}=4}\)
and
\(a-b=0\\a=b\\\boxed{\frac{a}{b}=1}\)