Limit of scaled differences of a differentiable function

Let $f:\mathbb R\to\mathbb R$ be a differentiable function such that $f'$ is continuous on $\mathbb R$ with $f'(3)=18$. Define

$$g_n(x)=n\left(f\left(x+\frac5n\right)-f\left(x-\frac2n\right)\right).$$

Then $\lim_{n\to\infty}g_n(3)$ equals ___________