2. \(\Rightarrow\left\{{}\begin{matrix}vtb\left(AB\right)=\dfrac{AB}{t1}=\dfrac{12}{\dfrac{1}{6}}=72km/h\\vtb\left(BC\right)=\dfrac{BC}{t2}=\dfrac{500}{3.60}=\dfrac{25}{9}m/s=10km/h\end{matrix}\right.\)
\(b,\Rightarrow vtb\left(AC\right)=\dfrac{12+\dfrac{500}{1000}}{t1+t2}=\dfrac{12,5}{\dfrac{1}{6}+\dfrac{3}{60}}=57,69km/h\)