This equation is in y=mx+b form, so to graph this equation, take the y-intercept which would be 12. Go on your graph and make twelve on the y-axis (you can go by two's or even six's). Then take your slope which would be -6 and use rise of run which would be down 6, since it's negative, and to the right one. Keep repeating this process until you ran out of space. Draw a line between the points and you have graphed a line.
