Copia de Average cost and marginal cost plotted on own axis

Aedifico Ltd is a small construction firm. The total cost (TC, in £) and marginal cost (MC) of Aedifico's concrete depends on tons produced (Q) as follows: TC = Q³ - 9Q² + 40Q + 50 MC = 3Q² – 18Q + 40 Marginal cost and average cost (TC/Q) have been plotted here. To see their relationship with the total cost curve, see
When is AC greater than MC? When is AC less than MC? Where is the smallest possible AC?