Answer
Let's just get the answer so that you see what we are trying to do with the distributive property.
Cost = C = n (number of books) * Cost of 1 book.
Cost = 7 * 11.20
Cost = 78.40
Distributive Property
11.20 * (4 + 3)
11.20 * 4 + 11.20 * 3
44.80 +33.60
78.40 which agrees with the multiplication answer