What would be the equation since it’s asking for horizontal stretch?

This is the number of parts out of 100, the numerator of a fraction where the denominator is 100. submit

What is the slope of the equation y =

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?