how do I do this???

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 $2.6 million decline in sales of Brand 2, 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 Z are each selling $6 million per year. Model the sales of Brand Z using a linear function. (Let 2 = annual sales on (in millions of dollars), x = annual sales of X (in millions of dollars), and y = annual sales of Y (in millions of dollars).) z