Let $f:\mathbb{R}\to\mathbb{R}$ be a differentiable function satisfying $f'(3)+f'(2)=0$. Then $\displaystyle \lim_{x\to0}\left(\frac{1+f(3+x)-f(3)}{1+f(2-x)-f(2)}\right)^{!1/x}$ is equal to: