\[\boxed{\mathbf{18.}}\]
\[\textbf{а)}\ 7 \cdot 2 = 14\]
\[7 \cdot 4 = 28\]
\[7 \cdot 6 = 42\]
\[35\ :7 = 5\]
\[28\ :7 = 4\]
\[49\ :7 = 7\]
\[\textbf{б)}\ 19 + 7 \cdot 3 = 19 + 21 = 40\]
\[7 \cdot 5 - 26 = 35 - 26 = 9\]
\[80 - 7 \cdot 8 = 80 - 56 = 24\]
\[91 - 14\ :7 = 91 - 2 = 89\]
\[56\ :7 + 92 = 8 + 92 = 100\]
\[90 - 63\ :7 = 90 - 9 = 81\]