You could found a lot of equivalent expressions for this.
In math, you could change and rearrange a lot for doing that.
In this case, the easiest way is to distribute the 5 with all of the other terms in the parenthesis.
[tex]5*(2x+y-3z)=[/tex]
[tex](5*2x)+(5*y)-(5*3z)=[/tex]
[tex]10x + 5y - 15z =[/tex] an this is an equivalent expression
