\[\boxed{\text{844\ (844).}\text{\ }\text{Еуроки\ -\ ДЗ\ без\ мороки}}\]
\[\textbf{а)}\ C_{9}^{4} = \frac{9!}{(9 - 4)!} = \frac{9!}{5!} =\]
\[= 6 \cdot 7 \cdot 8 \cdot 9 = 3024\ способа.\]
\[\textbf{б)}\ C_{9}^{3} = \frac{9!}{(9 - 3)!} = \frac{9!}{6!} =\]
\[= 7 \cdot 8 \cdot 9 = 504\ способа.\]