4-nitrotoluene is first brominated ortho to the nitro group (electrophilic substitution guided by $-NO_2$, using $Br_2$) to give 2-bromo-4-nitrotoluene; the nitro group is then reduced to amine ($Sn/HCl$); the amine is converted to a diazonium salt ($NaNO_2/HCl$); and finally the diazonium group is replaced by hydrogen using $H_3PO_2/H_2O$ (deamination), giving 2-bromotoluene. Sequence: $Br_2\to Sn/HCl\to NaNO_2/HCl\to H_2O/H_3PO_2$.