we know line P is y = 3x.
parallel lines have the same slope but different y-intercept, where the graph touches the y-axis. So then parallel lines is the same equation but shifted some either upwards or downwards or sideways.
so then line Q must be the same as P, y = 3x + "some change", well, we notice that the perpendicular distance from each other is 3 units, so then, the y-intercept of P is at (0, 0), thus the y-intercept of Q must be 3 units downwards, at (0, -3), since Q is shifted downwards, thus Q is y = 3x - 3.