Given that point a (x, 4-y) and point B (1-y, 2x) are symmetric about the Y axis, find the value of x-2y

Given that point a (x, 4-y) and point B (1-y, 2x) are symmetric about the Y axis, find the value of x-2y

X=Y-1
2X=4-Y
X=1
Y=2
X-2Y=-3