Here is one possible approach:

Start by considering concrete cases. Let N be the number of copies bought wholesale by Borders. Let P be the profit

N = 0: P = 0.

Therefore E(P) = 0.

N = 1: P = -0.4, 1.2.

Therefore E(P) = (-0.4) Pr(d = 0) + (1.2) Pr(d > 0) = ....

N = 2: P = -0.8, 0.8, 2.4.

Therefore E(P) = (-0.8) Pr(d = 0) + (0.8) Pr(d = 1) + (2.4) Pr(d > 1) = ....

N = 3: P = -1.2, 0.4, 2.0, 3.6.

Therefore E(P) = (-1.2) Pr(d = 0) + (0.4) Pr(d = 1) + (2.0) Pr(d = 2) + (3.6) Pr(d > 2) = ....

Continue in this way until you get to a value of N where E(P) is less than for the previous value of N.

(If you're clever enough to see a pattern in the above calculations, you can come up with a general formula for E(P) and then use a spreadsheet to calculate the values of E(P) for all values of N).