\[\boxed{\mathbf{975}\mathbf{.}\mathbf{\ }}\]
\[\frac{b \cdot 3}{4 \cdot 3} = \frac{3b}{12}\text{\ \ \ \ \ }\]
\[\frac{3b}{12} = \frac{21}{a}\ \]
\[a = 12.\ \ \ \]
\[3b = 21\]
\[b = 21\ :3\]
\[b = 7.\]
\[Ответ:x = 12;b = 7.\]