Place compass needle on external point R
Make compass width greater than distance from R to given line
Draw arc cutting given at 2 points
Mark / label points of intersection P and Q
Don't change compass width, move compass needle to Q, draw another arc below line crossing previous arc
Move compass needle to P , make arc below line
Mark / label point of intersection S.
Draw line from external point R to point where arcs intersect, S
Line RS is perpendicular to line PQ and passes thru external point R