Draw a line AB and draw another line CD parallel to AB at a distance of 3.5cm from it

1. draw a line segment AB.

2. at point A, draw a line perpendicular to AB.[ i.e. draw a right angle at A]

3. put the tip of compass at A, cut an arc of length 3.5 cm. let it intersect at C.

4. at point B, draw a right angle at B in the same direction of angle drawn in step 2.

5. put the tip of compass at B, cut an arc of length 3.5 cm. let it intersect at D.

6. join CD.

CD || AB

hope this helps you

**
**