\[px - 3py + 6 = 0;\ \ \ \ (1,5;\ - 1,5)\]
\[p \cdot 1,5 - 3p \cdot ( - 1,5) + 6 = 0\]
\[1,5p + 4,5p + 6 = 0\]
\[6p = - 6\]
\[p = - 1\]
\[Ответ:\ \ при\ \ p = - 1.\]