a. \(v'=s':t'=3:\dfrac{12}{60}=15\left(\dfrac{km}{h}\right)=\dfrac{25}{6}\left(\dfrac{m}{s}\right)\)
b. \(v_{tb}=\dfrac{s'+s''}{t'+t''}=\dfrac{3+\left[\left(5\cdot3,6\right)\cdot\dfrac{18}{60}\right]}{\dfrac{12}{60}+\dfrac{18}{60}}=16,8\left(\dfrac{km}{h}\right)\)