\[5\frac{1^{\backslash 7}}{12} - 3\frac{4^{\backslash 4}}{21} = 5\frac{7}{84} - 3\frac{16}{84} =\]
\[= 4\frac{91}{84} - 3\frac{16}{84} = 1\frac{75}{84} = 1\frac{25}{28}.\]