4x-2y=8 ==> y=2x-4 (to better show you what I mean)
2 lines are parallel if they have the same slope (m the coefficient of x).
So y=mx + b // y =2x-4 means m=2 & y=mx + b, becomes y=2x + b.
Moreover they are telling us that this function passes by (-2,1), where -2 represents x & 1, represents y. To calculate b , replace x & y by their values:
y=2x+b ==> 1 = 2(- 2) + b==> 1 = -4 + b ==> b= 5. Finally the equation is
y=-2x+5
