So, you can do this one of two ways.
The first is knowing what the numbers in your equation represent:
For example: y=mx+b in this case "b" is 9. b is the y-value when x=0. So the first point on our graph is (0,9). Next we have to pick an xvalue to solve for another y-coordinate.
I chose x= -3. Plug x into the equation to get y.
y=3x+9
y=3(-3)+9
y=(-9)+9
y=0
So, our second point is (-3,0).
connect the points with a ruler to graph the line.