Hi

I have a line with end points (x1,y1) and (x2,y2). If I rotate a line about the point (x3,y3) at an angle 'a', it wlll be parallel to x axis. At what angle should I rotate the line about another point (x4,y4), to make it parallel to the x axis?

Is there any formula for calculating the angle for rotating the line about the point (x4,y4)?

Manu