[tex]3^x+3^{x-1}+3^{x-2}+3^{x-3}=40|\cdot 3^3 \\27\cdot 3^x+9\cdot 3^{x}+3\cdot 3^x+3^x=1080\\3^x(27+9+3+1)=1080\\3^x\cdot 40=1080\\3^x=27\\3^x=3^3\\\boxed{x=3}[/tex]
3^(x-3) (3³+3²+3+1)=40
3^(x-3) (27+9+3+1)=40
3^(x-3) *40=40
3^(x-3)=1=3^0
x-3=0
x=3
