System of Equation word problem

A store sells cashews for $6.00 per pound and peanuts for $3.00 per pound. The manager decides to mix 30 pounds of peanuts with some cashews and sell the mixture for $4.00 per pound. How many pounds of cashews should be mixed with the peanuts so that the mixture will produce the same revenue as would selling the nuts separately?

The answer, from the back of the book, is 15 pounds of cashews.

This is what I've done so far..

Well you can't have negative amount of pounds, so this is wrong...