Find the marginal profit.

Suppose that the total profit in hundreds of dollars from selling x items is given by
P(x)= -x^2 + 8x - 14. Find the marginal profit at x= -1.
A. -200
B. 800
C. -700
D. 600
How do I solve it?

P'(x) = -2x + 8

I will evaluate at x = 1, since negative sales is not realistic.

P'(1) = -2(1) + 8 = 6, or $600.