Ta có:
$\begin{array}{l}
{v_{tb}} = \dfrac{s}{t} = \dfrac{s}{{{t_1} + {t_2}}} \\= \dfrac{s}{{\dfrac{s}{{2{v_1}}} + \dfrac{s}{{2{v_2}}}}} = \dfrac{1}{{\dfrac{1}{{2{v_1}}} + \dfrac{1}{{2{v_2}}}}}\\
\to 10,5 = \dfrac{1}{{\dfrac{1}{{2.12}} + \dfrac{1}{{2{v_2}}}}} \\\to {v_2} = 9,33km/h
\end{array}$