\[1)\ 9\ :\frac{3}{7} = \frac{9 \cdot 7}{3} = 21\]
\[2)\ a\ :10 = 21 \Rightarrow a = 210.\]
\[3)\ b\ :3 = 21 \Rightarrow b = 63.\]
\[Ответ:a = 210;\ \ b = 63.\]