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 $2.5 million decline in sales of brand z, whereas each $1 million increase in sales of brand y results in an increase of $0.9 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. (let z = annual sales of z (in millions of dollars), x = annual sales of x (in millions of dollars), and y = annual sales of y (in millions of

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