Brand Z$\text{'}$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\mathrm{.}3$ million decline in sales of Brand $Z,$ whereas each $$1$ million increase in sales of Brand $Y$ results in an increase of $$0\mathrm{.}8$ 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 dollars$).)$

$z=$

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