$\log _{3} 3 x+\log _{9} x=4$
\(\boxed{x = \sqrt{\frac{10^4}{3}}}\)
Step 1 :\(\frac{\log_{10} 3x}{\log_{10} 3} + \frac{\log_{10} x}{\log_{10} 9} = 4\)
Step 2 :\(\log_{10}(3x \cdot x) = 4\)
Step 3 :\(\log_{10}(3x^2) = 4\)
Step 4 :\(3x^2 = 10^4\)
Step 5 :\(x^2 = \frac{10^4}{3}\)
Step 6 :\(x = \pm \sqrt{\frac{10^4}{3}}\)
Step 7 :\(\boxed{x = \sqrt{\frac{10^4}{3}}}\)