x + 2y - 4= 0 (how to place the y on the other side)?

Simple,
x + 2y - 4 = 0
x - 4 = -2y
(x - 4 ) / -2 = y

2y = 4 - x
y = 2 - x/2

x + 2y - 4 = 0
x + 2y = 4
2y = 4 - x
y = (4 - x)/2
y = 4/2 - x/2
y = 2 - x/2

x + 2y - 4= 0
x - 4= - 2y

subtract 2y from both sides to isolate the 2y, so you get -2y=x-4 if you want only y then divide both sides by -2 to further simplify and you get y = -(x-4)/2

subtract 2y from both sides
x-4 = -2y