'A' and 'B' started a business by investing Rs. '4x' and Rs. '2x' respectively. Seven months later, 'A' withdrew Rs. 400 from his investment whereas 'B' invested Rs. 100 more. If at the end of the year, profit share of 'A' was Rs. 7,000 out of total profit of Rs. 21,000, then find the initial investment of 'B'.