`(2/3)^{x+1} + (2/3)^{x+3} = 104/243`
$\Leftrightarrow$ `(2/3)^{x} . (2/3)^{1} + (2/3)^{x} . (2/3)^{3} = 104/243`
$\Leftrightarrow$ `(2/3)^{x} . [(2/3)^{1} + (2/3)^{3}] = 104/243`
$\Leftrightarrow$ `(2/3)^{x} . [2/3 + 8/27] = 104/243`
$\Leftrightarrow$ `(2/3)^{x} . 26/27 = 104/243`
$\Leftrightarrow$ `(2/3)^{x} = 104/243 : 26/27`
$\Leftrightarrow$ `(2/3)^{x} = 4/9`
$\Leftrightarrow$ `(2/3)^{x} = (2/3)^{2}`
$\Leftrightarrow$ `x = 2`
$\text{Vậy}$ `x = 2`
