Assume the total cost of a firm is given by TC(L, K, R) = 2L2 +3K2 +4R2 +3LK +5LR+6KR-35L -39K -53R+54 where L, K, and R are the units of labor, capital and the raw materials (all in 1000 units) used in production. Find the combination of labor, capital and raw materials that minimizes the firm's total cost.