To simplify by distribution you multiply the term outside the parenthesis by the terms inside the parenthesis:
[tex]\begin{gathered} \frac{1}{2}(8-4g)=\frac{1}{2}\cdot8-\frac{1}{2}\cdot4g \\ \\ =\frac{8}{2}-\frac{4}{2}g \\ \\ =4-2g \end{gathered}[/tex]Then, the given expression simplified is 4 - 2g