First you have all ten books, but then every time after, you loose one because you can't put the same book in the group twice.
10 * 9 * 8 * 7 = 5040 groups
The scale factor applied to the model is 8000. 8000 times one equals 8000

Negative multiply negative = Positive
Negative multiply positive = negative

Thus the answer is -4a + 16b - 12c - 12d.