To find the slope given two points, you need to subtract the two y values over the two x values
(-5,-3) (9,-6)
-5 will be x1, and -3 will be y1. 9 will be x2, and -6 will be y2. You then need to put the numbers in the correct place in the equation.
y2-y1 -6-(-3) -9
-------- = -------- = ----
x2-x1 9-(-5) 14
Now that we have the slope (-9/14) you can use one of the coordinates to find the value of b.
Let's use (9,-6).
-6=-9/14(9)+b
-6=-81/14+b
+81/14
b=-3/14
Your final equation is y=-9/15x-3/14