\[\boxed{\text{641\ (641).}\text{\ }\text{Еуроки\ -\ ДЗ\ без\ мороки}}\]
\[\textbf{а)}\ py - p - 1 = 0\]
\[py = p + 1\ \ \ \ \ |\ :p\]
\[y = \frac{p + 1}{p}\]
\[1.\ p = 0 \Longrightarrow y\ \varnothing\]
\[2.\ p \neq 0 \Longrightarrow y = \frac{p + 1}{p}\]
\[Ответ:при\ p \neq 0:\ \ \ y = \frac{p + 1}{p}.\]
\[\textbf{б)}\ py - 3y - 4p + 12 = 0\]
\[y(p - 3) = 4p - 12\ \ \ \ \ \ \ |\ :(p - 3)\]
\[y = \frac{4p - 12}{p - 3}\]
\[1.\ p - 3 = 0,\]
\[\ \ p = 3 \Longrightarrow y - любое\ число.\]
\[2.\ p - 3 \neq 0\ \Longrightarrow\]
\[\Longrightarrow y = \frac{4 \cdot (p - 3)}{p - 3} = 4\ \]
\[Ответ:при\ p = 0:\]
\[y - любое\ число;\]
\[при\ p \neq 3:\ \ y = 4.\]