Steps of constructions:
1. Draw a line l mark a point on it.
2. With P as center draw an arc intersects the line at A and B.
3.With A as center and radius is more than AP draw an arc, with B as center and radius is more than AP draw another arc.
4. Mark the point of the intersection of the arcs is Q and join PQ
5. Mark a point X on PQ such that PQ=4 cm with help of compass.
6. with P as center draw an arc intersects l at C, PQ at D.
7.With X as center and same radius draw the arcs as before, intersects PQ at E.
8.Take the length of the arc CD and draw the arc at center E which cuts at F.
9. Draw a line passing through the points F and X.
Hence, the line m is required line.
Since, ∠FXE=∠CPE [Alternate interior angles are equal]
PX=4 cm