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