I keep getting zero for some reason?

Solve using co-factors and cramers rule:

A gift shop is making a mixture of almonds, pecans, and peanuts which cost $3.50 per pound, $4.00 per pound, and $2.00 per pound respectively. The store keeper wants to make 20 pounds of the mix to sell at $2.70 per pound. The number of pounds of peanuts is to be three times the number of pounds of pecans. Find the number of pounds of each to be used in the mixture.