[tex]\displaystyle\bf\\3^x+3^x:3=4\\\\3^x+3^x:3^1=4\\\\3^x+3^{x-1}=4\\\\3^{x-1}(3+1)=4\\\\3^{x-1}\times4=4~\Big |~:4\\\\3^{x-1}=1\\\\3^{x-1}=3^0\\\\x-1=0\\\\\boxed{\bf x=1}[/tex]