calculus – If \$f”(x)−f'(x)−f(x)−1=0\$ and \$f(0)=f(k)=0\$ what can we say about values that \$f(x)\$ takes over \$(0,k)\$?

Here $$f(x)$$ a twice-differentiable function that solves the differential equation $$f”(x)−f'(x)−f(x)−1=0$$ over $$R$$ and satisfies the condition $$f(0) = 0 = f(k)$$ for some $$k > 0$$. Does $$f(x)$$ take both positive and negative values over $$(0,k)$$ or does it take only positive or only negative values over this interval?