If $y = \mathrm{f}(x) + \mathrm{g}(x)$ then $\frac{dy}{dx}=\frac{d}{dx}\mathrm{f}(x)+\frac{d}{dx}\mathrm{g}(x)$.
In this case
- $\mathrm{f}(x) = \simplify{{a}x^{n_1}}$, so $\frac{d}{dx}\mathrm{f}(x) = \simplify{{a}*{n_1}x^{n_1-1}}$
- $\mathrm{g}(x) = \simplify{{b}x^{n_2}}$, so $\frac{d}{dx}\mathrm{g}(x) = \simplify{{b}*{n_2}x^{n_2-1}}$
So
$\frac{dy}{dx}= \simplify[all,!noLeadingMinus]{{a}*{n_1}x^{n_1-1} + {b}*{n_2}x^{n_2-1}}$
The second derivative is just differentiating again:
$\begin{align}\frac{d^2y}{dx^2}&= \frac{d}{dx}\left(\simplify[all,!noLeadingMinus]{{a}*{n_1}x^{n_1-1} + {b}*{n_2}x^{n_2-1}}\right)\\ &= \simplify[all,!noLeadingMinus]{{a}*{n_1}*{n_1-1}x^{n_1-2} + {b}*{n_2}*{n_2-1}x^{n_2-2}}\\ \end{align}$