Production function = F(L, K)= K + L

the Marginal rate of Technical Substituiton(MRTS) = MP_L/MP_k = 1 (MP = Marginal Product )

w_L/w_K   = 5/3 as, MP_L/MP_k <  w_L/wK ( w_L ,  w_K  are the input prices)

Hence, the firm will make use of capital(c) and they will avoid the usage of labour(L).

C= w_KK

let us now take from the production function:

Q = K

1 a) Total long run cost   C = 3Q

1 b) Average long run cost TC/Q = 3Q/Q = 3

1 c) Marginal long run cost dTC/dQ =3