Please help explain it?

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 $2.4 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.

right answer is z=-2.4x+0.9y+15
how to get the 15?

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