Slope-intercept form is y=mx+b, where m is the slope and b is the y-intercept.
If a line is parallel, it has the same slope. So this has a slope of 1/2. Then, you can plug in values for x and y to solve for b:
Y=mx+b
-1=(1/2)(8)+b
-1=4+b
b=-5
So the full equation is y=(1/2)x-5.
