Ответ:
\[21\left(\frac{1}{7}\right)^2-10\frac{1}{7}=21\cdot\frac{1}{49}-\frac{71}{7}=\frac{3}{7}-\frac{71}{7}=-\frac{68}{7}=-9\frac{5}{7}.\]
Ответ: \(-9\frac{5}{7}\).
\[21\left(\frac{1}{7}\right)^2-10\frac{1}{7}=21\cdot\frac{1}{49}-\frac{71}{7}=\frac{3}{7}-\frac{71}{7}=-\frac{68}{7}=-9\frac{5}{7}.\]
Ответ: \(-9\frac{5}{7}\).