Slope intercept form is y=mx+b, where x and y correspond to the coordinates on the coordinate plane, m is the slope, and b is the y-intercept.
b=-9 (the minus belongs to the y-intercept since the general form is "+b"
from this we have identified the y-intercept (0,-9). The x-coordinate of the y-intercept is always 0 since it is where it intersects the y-axis (x=0).
So the first point you graph is (0,-9). You need two points to draw a line so you need a second point. This is where the slope is useful. So the slope is 3, which means you go up three for every 1 you go over in the x direction. Since it is positive you go up three and one to the right (if it was negative you would go up and to the left, or down and to the right).
So up 3 and 1 to the right gets us (1, -6) as our second point. At this point you can use a straightedge to connect these points and continue in both directions and you have your line. If you prefer to draw more points first, you can continue with the slope up 3, right 1. (2, -3) (3, 0) (4, 3) Etc.