Find three points.  Actually two points is enough, as Jim Thompson
has shown you in his good explanation. Trouble is, any two points 
are such that a straight line can be drawn through both of them.  
And since people sometimes make mistakes we should get a third 
point as a check.  Then if you can't draw a straight line through 
all three of them, then you would know that there is a mistake in 
one of the three.

Let's make this table: 

  x | y | (x,y) 
 ---------------
    |   | ( , ) 
 --------------- 
    |   | ( , )
 ---------------
    |   | ( , )

Let's choose 0 for x for one point and choose 0 for y 
for a second point.  Such points are called the
"intercepts".


Then for the third check point we can choose any number 
positive or negative.  I'll arbitrarily choose 1 for x,
but you could choose any number for x or y.

  x | y | (x,y) 
 ---------------
  0 |   | (0, ) 
    | 0 | ( ,0)
  1 |   | (1, )

To find the value of y that corresponds to x=0, we substitute
0 for x in the equation:









So we fill in 3 for y, and we have:

  x | y | (x,y) 
 ---------------
  0 | 3 | (0,3) 
    | 0 | ( ,0)
  1 |   | (1, )

To find the value of x that corresponds to y=0, we substitute
0 for y in the equation:









So we fill in 2 for x, and we have:

  x | y | (x,y) 
 ---------------
  0 | 3 | (0,3) 
  2 | 0 | (2,0)
  1 |   | (1, )

To find the value of x that corresponds to x=1, we substitute
1 for x in the equation:











So we fill in 1.5 for x, and we have:

   x  |  y  | (x,y) 
 -------------------
   0  |  3  | (0,3)   
   2  |  0  | (2,0)   
   1  | 1.5 | (1,1.5)

   x  |  y  | (x,y) 
 -------------------
   0  |  3  | (0,3)   <--- that's the y-intercept 
   2  |  0  | (2,0)   <--- that's the x-intercept
   1  | 1.5 | (1,1.5)


Now we plot those three points:



Now we take a ruler and draw a line through them:



Edwin