I am having a hard time putting the differentiation equation together maybe someone could point me the in right direction. here is the problem : A particular factory manufactures n units per month. Each unit sells for c dollars. The number of units sold varies with the selling price according to n=600-10c and the monthly operating cost of the factory is given by C=4000+12n The monthly profit P=nc-C is the maximum when the unit selling price is (1) $42 (2) $36 (3) $420 (4) $90 I am just having a hard time getting the equation for it. I guess I am stuck deep just need a kick start. thanks for any help anyone could give me some advice.