It's always worth making the subject of the equation positive in the first instance, to avoid any silly errors by having to multiple by -1 at the end.
As I understand, the first figure to the right of the = sign will be a positive unless otherwise indicated,
Correct.
You ultimately want to get the subject on it's on in as few a "moves" (for lack of a better word!) as possible
So with this in mind....
y = 7 - 2x
I'm going to add 2x to each side of the equation (as I want to get the subject positive), alternativly you could multiply everything by -1 but I favour the first method.
y + 2x = 7
Then subract y from each side
2x = 7 - y (there is 0 y on this side of the equation hence why is becomes "-y" -y is the same as -1y)
then finally divide each side by 2 (as there are 2 lots of x on the left side. If we know that 7-y gives us twice the value of x, then half on 7-y must give us x!)
x = (7-y)/2