1. Do you mean maybe2. If not: If the line isn't parallel to the x-axis and you know one specific angleAt what angle should I rotate the line about another point (x4,y4), to make it parallel to theyaxis?which produces a parallel to the x-axis then any arbitrary point will serve as center of the rotation. You'll get a parallel to the x-axis, only the place, where this parallel will be, is changing.a

The line has the endpoints A and B;

the red point produces a parallel line to the x-axis (drawn in red)

an the blue point produces a parallel line to the x-axis different from the first parallel.

As you can see your anglehas the same value (but the opposite direction) as the angle formed by the original line and a parallel to the x-axis.a