One of my Calc Practice questions, I'm not sure what to do....any help is appreciated...

The total cost of manufacturing q hundred pencils is c dollars, where
c = 40 + 3q + \frac{q^2}{1000} + \frac{1}{q}
pencils are sold for $7 hundred, show that the break-even quantity is a solution of the equation f(q) = \frac{q^3}{1000} - 4q^2 + 40q + 1 = 0