$\mathrm{PhS}^{-}$is a strong nucleophile and dimethylformamide (DMF) is a highly polar aprotic solvent. Condition indicates that nucleophilic substitution $\left(\mathrm{S}_{\mathrm{N}} 2\right)$ takes place at $2^{\circ}$ benzylic place, stereochemically it involves inversion of configuration.
Aryl group can exhibit neighbouring group or can show anchimeric assistance but not possible here due to deactivity imparted by $-\mathrm{NO}_2$ and $-\mathrm{F}$. It can be if $-\mathrm{OCH}_3$ would have been instead of $-\mathrm{NO}_2$.
