t_{3} - t_{2} = \frac{s_{2}}{u} = \frac{5000 m}{10 \frac{m}{s}} = 500 s

${t_3 - t_2 =\frac{s_2}{u} = \frac{5000\,\mathrm{m}}{10\,\mathrm{\frac{m}{s}}} = 500\,\mathrm{s}}$