\[\boxed{\mathbf{12.}}\]
\[\textbf{а)}\ 2 \cdot 4 = 8\]
\[2 \cdot 6 = 12\]
\[2 \cdot 5 = 10\]
\[2 \cdot 7 = 14\]
\[\textbf{б)}\ 2 \cdot 2 \cdot 2 = 4 \cdot 2 = 8\]
\[2 \cdot 3 \cdot 3 = 6 \cdot 3 = 18\]
\[2 \cdot 9\ :3 = 18\ :3 = 6\]
\[2 \cdot 10\ :5 = 20\ :5 = 4\]