The marginal profit is the additional profit when one more copy is sold (this can be found either by subtracting the profit when 400 are sold from that when 401 are sold or by the derivative of the profit function with respect to sales, these should be approximatly the same).

The profit when x copies are sold is revenue-cost:

pr(x)=(2.3-0.003x)x - 20+0.4x

and you want: pr(401)-pr(400)

CB