[tex]3^{x} + 3^{x-1}+3^{x-2} +3^{x-3}=40 \\
3^{x-3+3} + 3^{x-3+2}+3^{x-3+1} +3^{x-3+0}=40 \\
3^{x-3} \times 3^{3} + 3^{x-3} \times 3^{2}+3^{x-3} \times 3^{1} +3^{x-3} \times 3^{0}=40 \\
3^{x-3}( 3^{3} + 3^{2}+ 3^{1} + 3^{0})=40 \\
3^{x-3}( 27 + 9+ 3 + 1)=40 \\
3^{x-3} \times 40=40 \\
3^{x-3} = \frac{40}{40} \\
3^{x-3} = 1 \\
3^{x-3} = 3^0 \\
x-3=0 \\
x=\boxed{3}[/tex]