The tangent of the angle between a straight line and the positive and semi-axis directions of the horizontal coordinate axis of the plane rectangular coordinate system is the slope of the straight line relative to the coordinate system.

Knowing this, it will be easy. We know that tan90 approaches infinity, so the slope does not exist when it is perpendicular to the X axis, and tan0 is 0, so the slope is 0 when it is parallel to the X axis.