how do you fill in the table for y=1/2 (x-8)

If you're referring to an x-y table then would you not just sub in a value for x and find the consequent value for y?

For example:

Lets sub in 0 for x
y= 1/2 (0-8)
y = 1/2 (-8)
y = -4

Lets sub in 1 for x
y= 1/2 (1-8)
y = 1/2 (-7)
y = -3.5

I believe this is what you're asking. You would simply do so for as many values as you'd like. I started at 0 but you could certainly go into negative x-values such as -1 or -2.