[tex]D:x \ \textgreater \ 0\\\\3\sqrt{x}=9x\ \ \ |square\ of\ both\ sides\\\\(3\sqrt{x})^2=(9x)^2\\\\9x=81x^2\ \ \ |divide\ both\ sides\ by\ 9\\\\x=9x^2\\\\9x^2-x=0\\\\x(9x-1)=0\iff x=0\ or\ 9x-1=0\\\\x=0\notin D\\\\9x-1=0\ \ \ |add\ 1\ to\ both\ sides\\9x=1\ \ \ |divide\ both\ sides\ by\ 9\\\boxed{x=\frac{1}{9}}\in D[/tex]
