\[10\frac{4}{21} - \left( 4\frac{10}{21} + 3\frac{19}{21} \right) =\]
\[= 10\frac{4}{21} - 7\frac{29}{21} =\]
\[= 9\frac{25}{21} - 8\frac{8}{21} = 1\frac{17}{21}.\]