A straight section of a displacement-time graph joins (t,s)=(2 s,18 m)(t,s)=(2\ \mathrm{s},18\ \mathrm{m})(t,s)=(2 s,18 m) and (6 s,2 m)(6\ \mathrm{s},2\ \mathrm{m})(6 s,2 m). Taking east as positive, what is the velocity?
5 m s⁻¹ west
4 m s⁻¹ east
4 m s⁻¹ west
5 m s⁻¹ east