[tex]y = ax + b[/tex]
[tex]\left\{ \begin{array}{lr} 2 = - a + b\\ 4 =a + b \end{array}\right. \\ b + b = 2 + 4 \\ 2b = 6 \: \: \: | \div 2 \\ b = 3 \\ 4 = a + 3 \\ a = 1 \\ \left\{ \begin{array}{lr} a = 1\\ b = 3 \end{array}\right.[/tex]
[tex]y = x + 3[/tex]
