An aromatic amine A ($\mathrm{C}{}_7\mathrm{H}{}_9\mathrm{N}$) gives a positive carbylamine test and a stable diazonium salt at 273-278 K. On treatment with excess bromine water A gives a tribromo derivative $\mathrm{C}{}_7\mathrm{H}{}_6\mathrm{Br}{}_3\mathrm{N}$. A is

Carbylamine means primary; a stable diazonium salt means aromatic NH2 on the ring, which rules out benzylamine. Excess Br2 water substitutes ortho and para to NH2 (positions 2, 4, 6). Three bromines can be taken up only if all three of these positions are free, which happens when the methyl group is at position 3. In o- and p-toluidine one of these positions is blocked and only two bromines enter.