\[\boxed{\mathbf{9.}}\]
\[\textbf{а)}\ 24 + 2 \cdot 6 = 24 + 12 = 36\]
\[18 - 12\ :2 = 18 - 6 = 12\]
\[2 \cdot 8 + 4 = 16 + 4 = 20\]
\[18\ :2 + 21 = 9 + 21 = 30\]
\[\textbf{б)}\ 78 + 4 \cdot 3 = 78 + 12 = 90\]
\[94 - 15\ :5 = 94 - 3 = 91\]