You do this by moving all terms without y to the right hand side, and then divide the whole lot to keep a clean y= on the left hand side.
Moving to the other side is done by subtracting (or adding) a term from both sides. Effectively this looks like you're moving the term and flipping the sign.
So 2y + 18 = 4x changes into 2y +18 -18 = 4x -18 so 2y = 4x - 18. If you forget about the middle step, it looks like the +18 jumped to the other side and changed sign. That's all.
Second step is to change the 2y into y. You do that by dividing everything by 2. So 2y=4x-18 becomes y=2x-9
