Question 839886
To graph a line choose two points. 
You can choose two x, two y, or one x and one y.
Use the equation to solve for the unknown.
Let {{{x=0}}}, solve for {{{y}}}
{{{x+y=-6}}}
{{{0+y=-6}}}
{{{y=-6}}}
(0,-6)
Let {{{y=0}}}, solve for {{{x}}}
{{{x+y=-6}}}
{{{x+0=-6}}}
{{{x=-6}}}
(-6,0)
Plot the points.
{{{drawing(300,300,-8,12,-18,2,grid(1),circle(-6,0,0.5),circle(0,-6,0.5),
graph(300,300,-8,12,-18,2,0))}}}
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Draw the line.
{{{drawing(300,300,-8,12,-18,2,grid(1),circle(-6,0,0.5),circle(0,-6,0.5),
graph(300,300,-8,12,-18,2,-6-x))}}}
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Similarly for {{{2x+y=4}}}
(0,4) and (2,0)
Now plot the points and connect them with a line.
{{{drawing(300,300,-8,12,-18,2,grid(1),circle(-6,0,0.5),circle(0,-6,0.5),
circle(0,4,0.5),circle(2,0,0.5),
graph(300,300,-8,12,-18,2,-6-x))}}}
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{{{drawing(300,300,-8,12,-18,2,grid(1),circle(-6,0,0.5),circle(0,-6,0.5),
circle(0,4,0.5),circle(2,0,0.5),circle(10,-16,0.35),
graph(300,300,-8,12,-18,2,-6-x,-2x+4))}}}
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Where the two lines intersect is the solution for the system of equations.