How do I get from the left side of this equation to the right?
Ok, let's see.
$\displaystyle \frac{d}{dx}[\frac{\sqrt{x^2+a^2}}{x}]=\frac{(x)\frac{d}{dx}[\sqrt{x^2+a^2}]-\sqrt{x^2+a^2}(\frac{d}{dx}x)}{x^2}$
$\displaystyle =\frac{(x)[\frac{1}{2}(x^2+a^2)^{(-\frac{1}{2})}(2x)]-\sqrt{x^2+a^2}\cdot 1}{x^2}=\frac{x(\frac{x}{\sqrt{x^2+a^2}})-\sqrt{x^2+a^2}\cdot 1}{x^2}$
Then multiply the fraction with the coefficient $\displaystyle -\frac{1}{a^2}$, you will have the desired result.
Roy