The cost function c(w1, w2, y) of a firm gives the cost of producing y units of output when the wage of factor 1 is w1 and the wage of factor 2 is w2. Find the cost functions for the following firms:

a. A firm with production function f (x1, x2) min2x1, 3x2

b. A firm with production function f (x1, x2) 2x1 3x2

c. A firm with production function f (x1, x2) max2x1, 3x2