\[\boxed{\text{75\ (75).}\text{\ }\text{Еуроки\ -\ ДЗ\ без\ мороки}}\]
\[\textbf{а)}\ ab + 3b - 5a - 15 =\]
\[= b(a + 3) - 5 \cdot (a + 3) =\]
\[= (a + 3)(b - 5)\]
\[\textbf{б)}\ 2xy - y + 8x - 4 =\]
\[= y(2x - 1) + 4 \cdot (2x - 1) =\]
\[= (2x - 1)(y + 4)\]