Find # of units when the marginal profit is 0

[hopelynnwelch]

I am given

c(x) = 11/2x^2-4x+7 and

p(x) = 5x^2-4x+2+3/x

I have to find:
a) Marginal Cost
b) Marginal Revenue
c) Marginal Profit and
d) the Quantity where Marginal Profit is Zero
This is where I need help.

Here is all my work.



a) c'(x) = 11x-4
b) R'(x) = 110x^3 -126x^2 + 124x - 21/(x^2) - 39/2
c) P'(x) = 110x^3 - 126x^2 + 113x - 31/2 -21/(x^2)
d) This is where I need help.

I set P'(x) = to 0
Does she expect me to solve this? If so how?

 

[hopelynnwelch]

I see what I did wrong. I was thinking p(x) was a quantity when the quantity is just x. This leads to a quadratic.