The marginal profit at x is defined as the instantaneous rate of change of profit with respect to the number x of items produced.

Suppose that the total profit in hundreds of dollars from selling x items is given by P(x)=4x^2-4x+1

The marginal profit when x=4 units are sold equals______ dollars/(unit of production).

I have attempted this question by taking the derivative of P(x)=4x^2-4x+1 which becomes P'(x)=8x-4 and plugged the x=4 into the derivative and got 28; however that is not the correct answer.

Can someone help me please.