Your slope is m = -1/3.
Explanation:
Using the slope formula

Select two points on the graph
I chose (-10, 6) and (-4,4)
Plug the numbers into the formula
Y2=6, Y1=4, X2=-10, X1=-4.

Simplify
=
or -0.33333333333333
Finally, find your b. Which is about 2.3