Momentum before: $$P_i=m_1v_1+m_2v_2$$

$$=(10000\text{ kg})(3.0\text{ m/s})+(10000\text{ kg})(0)$$

$$=30000\text{ kg$\cdot$m/s}$$

Momentum after: $$P_f=(m_1+m_2)v_f$$

where:

$$v_f=\frac{P_f}{m_1+m_2}$$

$$=\frac{30000\text{ kg$\cdot$m/s}}{(10000\text{ kg})+(10000\text{ kg})}$$

$$=1.5\text{ m/s}$$

Therefore, the combined cars have a speed of 1.5 m/s.