The easiest way to find the rate of change is to convert this equation into slope-intercept form, which is y = mx+b, with m = slope/rate of change and b = y-intercept.
Firstly, add 16 onto both sides of the equation:
Next, divide both sides by -2 and your slope-intercept form is
Now looking at the slope-intercept form, -8 is in the m placeholder, therefore making -8 the rate of change.