\[\boxed{\text{4.\ 313}}\]
\[\textbf{а)}\ (2a + 6a)\ :8 = 8a\ :8 = a\ \]
\[при\ a = - 14:\]
\[a = - 14;\]
\[при\ a = - 2,71:\]
\[a = - 2,71.\]
\[\textbf{б)}\ (4,7x - 4,7y)\ :4,7 = x - y\]
\[при\ x = - 36;y = - 4,67:\]
\[x - y = - 36 - ( - 4,67) =\]
\[= - 36 + 4,67 = - 31,33.\]