You need to open the brackets both sides such that:

`8x - 12y = 5x + 15y`

You need to isolate all terms in y to the left side and all terms in x to the right side such that:

`-12y - 15y = -8x + 5x`

`-27y = -3x`

Dividing by -27 both sides yields:

`y = (-3x)/-27 =gt y = x/9`

Hence, you should write y as a function of x such that: `y = f(x) = x/9.`