cob Douglas production function

Q 2: Given the following Cobb-Douglas production function: [4+3+3] Y = AL0.6 10.4 Where L and K represent labor and capital, respectively and cost of one unit of labor is "w" and cost of one unit of capital is "r". Answer the following: a) Derive cost function. b) Show that cross price substitution effects are positive [SLK = SKL] c) Show that WSLL + 1SLK = 0