x=price per book
profit P= -x2+25x-144
marginal profit estimate =(P(x+h)-P(x-h))/(2*h)
x	Profit		x	Marginal Profit Estimate
9.5	3.25		9.5
10	6		10	5
10.5	8.25		10.5	4
11	10		11	3
11.5	11.25		11.5	2
12	12		12	1
12.5	12.25		12.5	0
13	12		13	-1
13.5	11.25		13.5	-2
14	10		14	-3
14.5	8.25		14.5	-4
15	6		15	-5
15.5	3.25		15.5