Question

A 3 kg block moving to the right on a frictionless table at 4 m/s makes a head on collision with another 3 kg block moving 5 m/s to the left.

Answer 1

The law of conservation of momentum says

$$m_1v_1-m_2v_2=(m_1+m_2)u$$

The speed of blocks after collision

$$u=\frac{m_1v_1-m_2v_2}{m_1+m_2}=\frac{3*4-3*5}{3+3}=-0.5\:\rm m/s$$

The velocity is directed leftward.