\[\boxed{\mathbf{6.}}\]
\[\textbf{а)}\ 7 \cdot 2 = 14\]
\[7 \cdot 5 = 35\]
\[7 \cdot 6 = 42\]
\[4 \cdot 6 = 24\]
\[5 \cdot 9 = 45\]
\[3 \cdot 7 = 21\]
\[21\ :3 = 7\]
\[48\ :4 = 7\]
\[40\ :5 = 8\]
\[\textbf{б)}\ 11 + 7 \cdot 7 = 11 + 49 = 60\]
\[7 \cdot 9 - 15 = 63 - 15 = 48\]
\[100 - 4 \cdot 7 = 100 - 28 = 72\]