\[\boxed{\text{399\ (399).}\text{\ }\text{Еуроки\ -\ ДЗ\ без\ мороки}}\]
\[\textbf{а)}\ 3x + 0y = 12\]
\[3x = 12\]
\[x = 4.\]
\[\textbf{б)}\ 0x + y = 1\]
\[y = 1.\]
\[\textbf{в)}\ x = 5\ \]
\[\textbf{г)}\ y = 1,5\]
\[\textbf{д)}\ (x - 2)(y - 3) = 0\]
\[x = 2;\ \ \ y = 3.\]
\[\textbf{е)}\ (x + 3)(y + 1) = 0\]
\[x = - 3;\ \ y = - 1.\]
\[\textbf{ж)}\ |x| = 2\]
\[x = 2;\ \ \ x = - 2.\]
\[\textbf{з)}\ |y| = 3\]
\[y = 3;\ \ y = - 3.\ \]