Solution (source code)

= Solution

The numerator and denominator both vanish at zero. The <fundamental theorem of calculus> differentiates the numerator, while the denominator <derivative> is nonzero near zero. <L'Hôpital's rule> therefore gives
$$
\boxed{\lim_{x\to0}\frac{\int_0^xe^{-y^2}dy}{3x-x^3}=\lim_{x\to0}\frac{e^{-x^2}}{3-3x^2}=\frac13.}
$$