Let's draw the point (-2,-3):



Now draw a horizontal line through the point (-2,-3)



Now let's find another point on the line:



That point has coordinates (4,-3).



Now let's find one more point on the line:



That point has coordinates (1,-3).



Now look at those three points on that line:

(-2,-3), (4,-3), (1,-3)

Notice that every one of those points has its
y coordinate equal to -3.  You can also see
that no matter how many points we get on that
line, every one of them will have -3 for its 
y-coordinate. 

So you merely
write that fact as "y always equals -3" 

and so it's equation is simply:

y = -3.

Edwin