$\text{Đáp án+Giải thích các bước giải:}$
$\text{$\frac{21}{28}$ - $\frac{54}{72}$ : ($\frac{378}{504}$ + $\frac{336}{504}$)}$
$\text{= $\frac{21}{28}$ - $\frac{54}{72}$ : $\frac{17}{12}$}$
$\text{= $\frac{21}{28}$ - $\frac{54}{72}$ . $\frac{12}{17}$}$
$\text{= $\frac{21}{28}$ - $\frac{9}{17}$}$
$\text{= $\frac{357}{476}$ - $\frac{252}{476}$}$
$\text{= $\frac{105}{476}$ = $\frac{15}{68}$}$