When light ray is incident on the first mirror parallel to the second mirror, the angle of incidence at first mirror becomes $30°$.

From the laws of reflection, we know that the angle of reflection and angle of incidence are equal. So, the light ray reflects from the first mirror and is incident on the second mirror with an angle of incidence $30°$ and finally reflects, as shown in figure. As final reflected ray makes an angle of  $60°$ with the second mirror then i can be concluded that reflected ray from second mirror is parallel to first mirror.