\[\frac{2}{28}=\frac{1}{14},\qquad \frac{15}{28}:\frac{10}{21}=\frac{15}{28}\cdot\frac{21}{10}=\frac{9}{8}.\]
\[\frac{1}{14}-\frac{9}{8}=\frac{4}{56}-\frac{63}{56}=-\frac{59}{56}.\]
Ответ: \(-\frac{59}{56}\).