\[y = kx + l;\ \ A(0;5);\ \ B(2;1)\]
\[5 = k \cdot 0 + l\]
\[l = 5 \Longrightarrow y = kx + 5.\]
\[1 = k \cdot 2 + 5\]
\[2k = - 4\]
\[k = - 2 \Longrightarrow y = - 2x + 5.\]
\[Ответ:y = - 2x + 5.\]