assume we have a line segment of known length.
1. Draw a line and mark a point on it lable it b.
2. Mark off a length of 7cm from a alonf the line mark the end point c (will use your segment of known length, you should know how to do this)
3. At b construct a right angle, produce the line upwards.
4. At c construct a 60 degree angle of the appropriate orientation, now bisect it, and produce the line towards the line in 3 above.
5. Mark the point at which these lines meet a, and you are done.
Find a pt d on ac such that it is equidistant from both sides ab and bc .measure db.