Solve the equation for y.

y-4=1/2 (x-8)

All that's necessary is to add 4 to both sides of the equation. This gives

... y = 1/2 (x - 8) + 4

If you like, you can simplify this by eliminating the parentheses and collecting terms.

... y = 1/2·x - 4 + 4

... y = (1/2) x