Well that is true up until xy - 15y - 17 = x

From there you need to subtract x from both sides and add 15y and 17 to both sides.

That gives you xy - x = 15y + 17
Then factor out the x which gives you x(y - 1) = 15y + 17
Then divide both sides by (y - 1)


x = (15y + 17)/(y - 1)

What is it they say about silence being golden?