Tomamos como el límite
$$\lim_{x \to 3^+}\left(\frac{x \left(10 - 5 x\right)}{- x + \left(x^{2} + 4\right)}\right)$$
cambiamos
$$\lim_{x \to 3^+}\left(\frac{x \left(10 - 5 x\right)}{- x + \left(x^{2} + 4\right)}\right)$$
=
$$\lim_{x \to 3^+}\left(\frac{\left(-1\right) 5 x \left(x - 2\right)}{x^{2} - x + 4}\right)$$
=
$$\lim_{x \to 3^+}\left(- \frac{5 x \left(x - 2\right)}{x^{2} - x + 4}\right) = $$
$$- \frac{15 \left(-2 + 3\right)}{- 3 + 4 + 3^{2}} = $$
= -3/2
Entonces la respuesta definitiva es:
$$\lim_{x \to 3^+}\left(\frac{x \left(10 - 5 x\right)}{- x + \left(x^{2} + 4\right)}\right) = - \frac{3}{2}$$