\[\boxed{\mathbf{750\ (750)}\mathbf{.}\mathbf{\ }Еуроки\ - \ ДЗ\ без\ мороки}\]
\[\textbf{а)} - 3 + 8 + 7 + ( - 4) = \left( - 3 + ( - 4) \right) + (8 + 7) = - 7 + 15 = 8\]
\[\textbf{б)}\ 15 + ( - 5) + 0 + ( - 12) + ( - 7) = 15 + \left( ( - 5) + ( - 12) + ( - 7) \right) =\]
\[= 15 + ( - 24) = - 9\]