Steps of construction :
1. Draw a line AB = 6.0 cm.
2. With A as centre and a radius equal to more than half of AB draw two arcs on both sides of line AB.
3. With B as centre draw and with same radius draw two arcs on both sides of AB which cut previous arcs at C and D respectively.
4. Join C D, which intersect AB at point X. X will be the mid-point of AB.
Then, CD is the required line.