If $\mathrm{f}^{\prime}(\mathrm{x})<0 \forall \mathrm{x} \in(\mathrm{a}, \mathrm{b})$ and c is a point such that $\mathrm{a}<\mathrm{c}<\mathrm{b}$, and (c, $\left.\mathrm{f}(\mathrm{c})\right)$ is the point lying on the curve for which $\mathrm{F}(\mathrm{c})$ is maximum, then $f^{\prime}(c)$ is equal to