F(x) =
-**
2x2 + 6x + 4 if x < -4
6 – x2 if -4 < x < 12
14 if x > 12

Last month, a 770 g box of cereal was sold at a grocery store for $3.10. however, this month, the cereal manufacturer launched the same cereal in a 640 g box, which is being sold at $2.50. what is the percent change in the unit price?