Kool Klothes has determined that the revenue function for selling x thousand pairs of shorts is R(x) =-5x^2 + 21x. The cost function C(x) = 2x + 10 is the cost of producing the shorts.

Question

Kool Klothes has determined that the revenue function for selling x thousand pairs of shorts is R(x) =-5x^2 + 21x. The cost function C(x) = 2x + 10 is the cost of producing the shorts. 
How many pairs of shorts must the company sell in order to break even

Steve · Answer

you want cost = revenue

2x+10 = -5x^2+21x
5x^2 - 19x + 10 = 0

I think that R(x) is bogus, since it starts to decline as more shorts are sold.

http://www.wolframalpha.com/input/?i=2x%2B10+%3C+-5x%5E2%2B21x