$$\frac{1}{f} = \frac{n_2-n_1}{n_1\left(\frac{1}{R_1}-\frac{1}{R_2}\right)}$$
How do I isolate $R_2$ onto the left side of the equation? So far, I know that I have to cross multiply first and then subtract $1/R_1$ on both sides. Then subtract $1/R_1$ from $f(n_2-n_1)/n_1$. However I'm confused as to what to do next. Could someone please explain step by step on how to get the answer? Thank you!