‘P’ and ‘Q’ started a business by investing Rs. 4,000 and Rs. 6,000 respectively. ‘x’ months later, they admitted ‘R’ as a partner who invested Rs. 8,000. If at the end of the year, the profit share of ‘P’ was Rs. 6,000 out of the total profit of Rs. 18,000, then find the value of ‘x’.