Write the equation of the line passing through 
(3, –7) and (–6, –7). 

Use the slope formula:

     y2 - y1
m = —————————
     x2 - x1

where (x1, y1) = (3, -7) and (x2, y2) = (-6, -7)

     (-7) - (-7)     -7 + 7      0
m = ————————————— = ———————— = ———— = 0
      (-6) - (3)     -6 - 3     -9 

Now substitute in the point slope formula:

   y - y1 = m(x - x1)

   y - (-7) = (0)( x - (-6) )

      y + 7 = 0

          y = -7

which is a horizontal line 7 units below
and parallel to the x-axis.

You could have just plotted the points and
drawn a line through them,
 

and observed that every point on the line 
has the y-value -7, and just immediately 
written that y always = -7, or y = -7.

Edwin