You have a good idea. But you need to define variables properly. Let $$ y=\log_{\frac{1}{3}}(3^{2x}) $$ Then $$ 3^{-y}=\left(\frac{1}{3}\right)^y = 3^{2x}. $$ The exponents in the previous equation must be the same (since the exponential function is injective). Thus $$ y=-2x. $$