Centre of Mass and Collision Part 1

Two blocks of masses 10‌kg and 30‌kg are placed on the same straight line with coordinates (0,0)‌cm and (x,0)‌cm respectively. The block of 10‌kg is moved on the same line through a distance of 6‌cm towards the other block. The distance through which the block of 30‌kg must be moved to keep the position of centre of mass of the system unchanged is :