Let's plot those two points:



As you see those two points are on the y-axis, so if we connect
them, the line will coincide with the y-axis -- in fact the line 
IS the y-axis.  So we want the equation of the y-axis.  Look at 
a couple more points on that line, the y-axis, say (0,5) and (0,-7). 



Notice that all those point have 0 for their x-coordinate.  Every point
on the y-axis has its x-coordinate 0, so the way the equation is made
is to state that fact in an equation about all the x-coordinates being
0.  The equation that says that is:

     x = 0

That is the equation of the y-axis, because every point on the y-axis
has its x coordinate as 0.

Edwin