Question 549972
To graph a line, all you have to do is determine two points, plot them, and connect them.
Line {{{y=x}}}
For {{{x=0}}}, {{{y=x=0}}}, giving us the point (0,0).
For {{{x=6}}}, {{{y=x=6}}}, giving us the point (6,6).
Line {{{y=2x}}}
For {{{x=0}}}, {{{y=2x=0}}}, giving us the point (0,0).
For {{{x=3}}}, {{{y=2x=2*3=6}}}, giving us the point (3,6).
Line {{{ x + y = 6}}}
For {{{x=0}}}, {{{x + y=0+y=y=6}}}, giving us the point (0,6).
For {{{y=0}}}, {{{x + y=x+0=x=6}}}, giving us the point (6,0).
Plotting all 3 pairs of points, and connecting each pair to make the corresponding line, we get:
{{{drawing(300,300,-1,7,-1,7,
grid(1),line(-1,-1,7,7),line(-1/2,-1,3.5,7),line(-1,7,7,-1)
)}}} The vertices appear to be (0,0), (2,4), and (3,3).
We know that (0,0) is a point in the first two lines (we used that point to graph them).
We should verify that (2,4) and (3,3) are indeed at the intersection of line {{{x+y=6}}} with the first two lines, but that is so mental-math obvious that I'm going to skip it and proclaim that the three vertices are indeed (0,0), (2,4), and (3,3).