The first step is anti addition of ${Br}_2$ across the double bond.



In the second step, $Et{O}^{-}$ $(n{u}^{-}$ ) gets added by ${\mathrm{S}}_{\mathrm{N}}1$ mechanism. And $B{r}^{-}$ from the dashed position acts as the leaving group leads to the formation of a stable benzylic carbocation.