`( 1/3 )^( x + 3 ) + ( 1/3 )^x = 28/243`

`=> ( 1/3 )^x . ( 1/3 )^3 + ( 1/3 )^x . 1 = 28/243`

`=> ( 1/3 )^x . [( 1/3 )^3 + 1 ] = 28/243`

`=> ( 1/3 )^x . [ 1/27 + 1 ] = 28/243`

`=> ( 1/3 )^x . 28/27 = 28/243`

`=> ( 1/3 )^x = 28/243 : 28/27`

`=> ( 1/3 )^x = 1/9`

`=> ( 1/3 )^x = ( 1/3 )^2`

`=> x = 2`

Vậy `x = 2`

`#`Nguyencanhthinh`#`
`*** Hoidap247`