Question 184270
To graph a line, all you need are two (x,y) points.  
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For your first equation:
{{{ y = 3x }}}, substitute in a few numbers for x and see what you get for y:
If x = 0, then y = 3(0) = 0.  That gives us the point (0, 0).
If x = 2, then y = 3(2) = 6.  That gives us the point (2, 6).
Now we can plot those points and draw the line through them:
{{{graph(300,200,-5,5,-8,8,3x)}}}
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For your other equation:
{{{ y = -x + 4 }}}, substitute in a few numbers for x and see what you get for y:
If x = 0, then y = -(0) + 4 = 4.  That gives us the point (0, 4).
If x = 2, then y = -(2) + 4 = 2.  That gives us the point (2, 2).
Now we can plot those points and draw the line through them:
{{{graph(300,200,-5,5,-8,8,-x + 4)}}}
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Hope this helps. ~ Joe