First you should rearrange to get it in the form y = mx + c, and you do this by subtracting the 9x from both sides and dividing by 15, which gives you:
y = (-9/15)x + 3
Now you know the y-intercept is 3, so you can mark that on as a point.
Then, to graph the gradient of -9/15, you go along 15 spaces to the right and 9 spaces down, or 15 to the left and 9 spaces up, and you repeat this until you run out of room. Then you just join the dots
I hope this helps! Let me know if you have any questions :)
