Đáp án:
$\begin{array}{l}
y = ax + b\\
a)\text{Hệ số góc}:k = a = - 2\\
\Rightarrow y = - 2x + b\\
Do:A\left( { - 1;2} \right) \in y = - 2x + b\\
\Rightarrow 2 = - 2.1 + b\\
\Rightarrow b = 4\\
\text{Vậy}\,y = - 2x + 4\\
b)\text{Tung độ gốc}:b = 3\\
\Rightarrow y = a.x + 3\\
\left( { - 1;0} \right) \in y = a.x + 3\\
\Rightarrow 0 = a.\left( { - 1} \right) + 3\\
\Rightarrow a = 3\\
\Rightarrow y = 3x + 3\\
c)B\left( {1;2} \right);C\left( {3;6} \right) \in y = a.x + b\\
\Rightarrow \left\{ \begin{array}{l}
2 = a.1 + b\\
6 = a.3 + b
\end{array} \right.\\
\Rightarrow \left\{ \begin{array}{l}
a + b = 2\\
3a + b = 6
\end{array} \right.\\
\Rightarrow \left\{ \begin{array}{l}
2a = 4\\
b = 2 - a
\end{array} \right.\\
\Rightarrow \left\{ \begin{array}{l}
a = 2\\
b = 0
\end{array} \right.\\
\Rightarrow y = 2x
\end{array}$
