$3^{x-1}$ `+` $3^{x-2}$ `+` $3^{x-3}$ `=` `1053`
$3^{x}$ `÷` `3/1` `+` $3^{x}$ `÷` `3²/1` `+` $3^{x}$ `÷` `3³/1` `=` `1053`
$3^{x}$ `×` `1/3` `+` $3^{x}$ `×` `1/3²` `+` $3^{x}$ `×` `1/3³` `=` `1053`
$3^{x}$ `×` `(``3/1` `+` `{3²}/{1}` `+` `{3³}/1``)` `=` `1053`
$3^{x}$ `×` `13/27` `=` `1053`
$3^{x}$ `=` `1053` `÷` `13/27`
$3^{x}$ `=` `2187`
$3^{x}$ `=` `3^7`
⇒ `x` `=` `7`