It is false that deserts advance linearly, in easily mappable patterns.
Deserts don't really have a pattern when they are advancing - there are so many ways they can do this.
First thing to do is to solve each of these for y. The first one is y=-4x-3; the second one is y=4x-21; the third one is y=4x+21; the fourth one is y=-4x+3. From that you can tell the positive slopes are found in the second and third equations. Those are the ones we will test now for the point (3, -9). y=-9 and x=3, so let's fill in accordingly. The second equation filled in is -9=4(3)-21. Does the left side equal the right when we do the math? -9=12-21 and -9=-9. So the second one works. Just for the sake of completion, let's do the same with the third: -9=4(3)+21. Does -9=12+21? Of course it doesn't. Our equation is the second one above, y+9=4(x-3).
These tables have infinitely many values, but the simplest ones would be:
a) x|y
3|5
6|10
9|15
b) x|y
2|1
4|2
6|3
Your graph for c would look like the attached picture
