According to Ohm's law, the voltage V, current I, and resistance R in a circuit are related by the equation V=IR, where the units are volts, amperes, and ohms. Assume that voltage is constant with V=12V. Calculate:

The ROC of I with respect to R for the interval from R=8 to R=8.1

So I found
I'(R)=-12/R^2

Now what?