Answer
The particle that was travelling along the solid green line before the collision, travels along the dashed orange line after the collision.
The particle that was travelling along the solid orange line before the collision, travels along the dashed green line after the collision.
Work Step by Step
According to Equations (9-75) and (9-76), when $m_1=m_2$ in an elastic collision, then $v_{1f} = v_{2i}$ and $v_{2f} = v_{1i}$
That is, the velocities are exchanged after the collision.
Therefore:
The particle that was travelling along the solid green line before the collision, travels along the dashed orange line after the collision.
The particle that was travelling along the solid orange line before the collision, travels along the dashed green line after the collision.
