The Chain Rule & Implicit Differentiation · Medium problem

Two resistors are connected in parallel, so the total resistance $R$ satisfies $\dfrac1R = \dfrac1{R_1} + \dfrac1{R_2}$. At a certain moment $R_1 = 3$ ohms and is increasing at $0.3$ ohm/s, while $R_2 = 6$ ohms and is decreasing at $0.6$ ohm/s. How fast is $R$ changing at that moment, in ohms per second?