Answer
(a) $-\frac{2}{3}$
(b) $4$
Work Step by Step
(a) $\frac{dI}{dR}= \frac{d}{dR}(\frac{V}{R})=V\frac{d}{dR}(\frac{1}{R})=V\times-\frac{1}{R^{2}}$
As $V=24$, $\frac{dI}{dR}|_{R=6}=24\times-\frac{1}{6^{2}}=-\frac{2}{3}$
(b) $\frac{dV}{dR}=\frac{d}{dR}(IR)=I\frac{dR}{dR}=I\times1=I=4$
