Brand Z's annual sales are affected by the sales of related products X and Y as follows: Each $1 million increase in sales of brand X causes a $2.8 million decline in sales of brand Z, whereas each $1 million increase in sales of brand Y results in an increase of $0.1 million in sales of brand Z. Currently, brands X, Y, and Z are each selling $6 million per year. Model the sales of brand Z using a linear function. (Letz= annual sales ofZ(in millions of dollars),x= annual sales ofX(in millions of dollars), andy= annual sales ofY(in millions of dollars).)