\[а)\ y = kx;\ \ \ (2;1)\]
\[2k = 1\ \ \]
\[k = \frac{1}{2} = 0,5\]
\[Формула:\ \ y = 0,5x.\]
\[б)\ Формула:\ y = 2.\]
\[в)\ y = kx + b;\ \ \ \ \ (2;0);\ (0;3)\]
\[2k + b = 0\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ 0k + b = 3\]
\[2k + 3 = 0\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ b = 3\]
\[k = - 1,5\]
\[Формула:\ y = - 1,5x + 3\ \]