\[3a - 15b\]
\[a = - 3\ и\ b = - 7:\]
\[3 \cdot ( - 3) - 15 \cdot ( - 7) =\]
\[= - 9 + 105 = 96.\]
\[a = - \frac{3}{4}\ и\ b = 0,8:\]
\[3 \cdot \left( - \frac{3}{4} \right) - 15 \cdot 0,8 = - \frac{9}{4} - 12^{\backslash 4} =\]
\[= - \frac{9}{4} - \frac{48}{4} = - \frac{57}{4} =\]
\[= - 14\frac{1}{4} = - 14,25.\]