How do I graph a line when rise=x?

Question

I have the inequality $x+3y<21$. I have rearranged to y-intercept form to give me $y<-x/3+7$. I must have missed something in learning to graph in this form because I'm very unsure on how to graph a line when slope =$x$.

Answer 1

$$-x/3 = -\frac x3 = -\frac13 x$$

So $y < -x/3 + 7$ is the same as $y < -\dfrac13x + 7$.