To calculate the length of a spring with a 2 kg load, compare the displacement of a 1 kg load and adjust accordingly.
When a 1 kg load is suspended from the spring, the spring which is 6 cm stretches to 9 cm. This is 3 cm longer due to the weight. If you attach a weight which is twice as much then the displacement will be twice as much. Instead of stretching an additional 3 cm, it will stretch 2*3 = 6 cm. Add this to the length of the spring and it stretches in total 6 + 6 = 12 cm.
