Given that the solution of the initial system of equations is (1,4), we need to find a third line such that it goes through (1,4); then,
[tex]\begin{gathered} y=ax+b \\ and \\ (1,4) \\ \Rightarrow4=a+b \end{gathered}[/tex]
We can set any value for a; for example, a=1; thus,
[tex]\begin{gathered} a=1 \\ \Rightarrow4=1+b \\ \Rightarrow b=3 \end{gathered}[/tex]
Therefore, one possible third line is
[tex]y=x+3[/tex]
The green line is the one you need to graph, the others have already been plotted.
