Olá, Monica!
a)
Basta fazer:
[tex3]Q_{a} \, = \, Q_{d} \\ m_{1} \cdot v_{1} \, + \, m_{2} \cdot v_{2} \, = \, \left(m_{1} \, + \, m_{2}\right)V \,\,\, \Rightarrow \,\,\, V \, = \, \frac{m_{1} \cdot v_{1} \, + \, m_{2} \cdot v_{2}}{m_{1} \, + \, m_{2}} \\ \\ V \, = \, \frac{1200 \cdot 25 \, + \, 9000 \cdot 20}{10 \, 200} \,= \, \boxed{\boxed{20,59 \,\, m\cdot s^{-1}}}[/tex3]...