Graph $E(y)$ versus $x_{1}$ for values of $x_{2}=0,1,$ and 2 on the same sheet of paper. Now graph $E(y)$ versus $x_{2}$ for values of $x_{1}=0$, l, and 2 on another sheet of paper. What relationships do the three lines on the graphs have to one another?
$$E(y)=1-2 x_{1}+3 x_{2}$$