The difference of x and y is written as [tex] x-y [/tex].
Now you have to take 8 times this difference, which means that you have to multiply the whole expression by 8. You'll need parenthesis: you have to write
[tex] 8(x-y) [/tex]
In fact, if you wrote
[tex] 8x-y [/tex]
the 8 would multiply x alone, instead of the whole difference.
