\[\boxed{\text{211\ (211).}\text{\ }\text{Еуроки\ -\ ДЗ\ без\ мороки}}\]
\[2 \cdot (a + b) = - 8,1\]
\[a + b = - 8,1\ :2\]
\[a + b = - 4,05.\]
\[\textbf{а)}\ 3 \cdot (a + b) = 3 \cdot ( - 4,05) =\]
\[= - 12,15.\]
\[\textbf{б)} - 0,5 \cdot (a + b) =\]
\[= - 0,5 \cdot ( - 4,05) = 2,025.\]
\[\textbf{в)}\ 4a + 4b = 4 \cdot (a + b) =\]
\[= 4 \cdot ( - 4,05) = - 16,2.\]
\[\textbf{г)} - 5a - 5b = - 5 \cdot (a + b) =\]
\[= - 5 \cdot ( - 4,05) = 20,25\]