Let's plot the point (2,1):



Next let's draw a horizontal line through it (in green):



Now let's plot another point on that line, say the point
(-3,1)



Now we use the slope formula:











Now use the slope-intercept form:









A much easier way is just to observe that every point
on that horizontal line will have the same y-coordinate,
so since the point (2,1) has the y-coordinate 1, that means
all values of y on this line will also be 1.  So we could
just have written down the equation

y = 1

in the beginning in one step!!!

Edwin