A company has found that if it produces x thousand units/day of a product, The total cost would be T(x) thousand $, where T(x) = x^3 - 6x^3 + 13x + 15, when 0<x<9. They sell it for 28 $/unit.

a) State the profit V(x) thousand $ as a function of x.

b) Find the maximum profit per day.