Brand Z's annual sales are affected by the sales of related products $\mathrm{X}$ and $\mathrm{Y}$ as follows: Each $$\$ 1$$ million increase in sales of brand $X$ causes a $$\$ 2.1$$ million decline in sales of brand $Z$, whereas each $$\$ 1$$ million increase in sales of brand Y results in an increase of $$\$ 0.4$$ million in sales of brand $Z$. Currently, brands X, Y, and $\mathrm{Z}$ are each selling $$\$ 6$$ million per year. Model the sales of brand $\mathrm{Z}$ using a linear function.