Tomamos como el límite
$$\lim_{x \to 0^+}\left(\frac{- a^{2} + x^{2}}{- a^{3} + x^{3}}\right)$$
cambiamos
$$\lim_{x \to 0^+}\left(\frac{- a^{2} + x^{2}}{- a^{3} + x^{3}}\right)$$
=
$$\lim_{x \to 0^+}\left(\frac{\left(- a + x\right) \left(a + x\right)}{\left(- a + x\right) \left(a^{2} + a x + x^{2}\right)}\right)$$
=
$$\lim_{x \to 0^+}\left(\frac{a + x}{a^{2} + a x + x^{2}}\right) = $$
$$\frac{a}{a^{2} + 0 a + 0^{2}} = $$
= 1/a
Entonces la respuesta definitiva es:
$$\lim_{x \to 0^+}\left(\frac{- a^{2} + x^{2}}{- a^{3} + x^{3}}\right) = \frac{1}{a}$$