The rate of change here, is the how much the value of the refridgerator increases per unit of time, in years. Since, the value of the fridge decreases, the rate of change is negative.
It is a linear graph i.e. the instantaneous rate of change and average rate of change are the same. Now, all you have to do is pick any two arbitrary points, preferably the x-intercept and y-intercept for simplicity and find the change in y and the change in x. The change in y and x will be the difference of the x and y coordinate of the points you chose. It does not matter which order you subtract in. Just remain consistent between your 2 points. The change in y is your rise, the change in x is your run. Slope is rise over run. So then divide.
Δy=400-0= $400
Δx = 0-12 = -12 yrs
Rate of change = rise/run = 400/(-12)=100/-3= -33.333 $/yrs
