Steps for construction,
1. Draw a line AB.
2. Take any two points P and Q on AB.
3. Construct angle BPE = angle BQF= 90o
4. With P as center and radius equal to 3.5cm, cut PE at R.
5. With Q as center and radius equal to 3.5cm, cut QF at S.
6. Join CD and produce it on either side to get the required line CD parallel to AB and at a distance of 3cm from it.