well, we can say that the function will be y = (50% of "x") - 8.
wait a minute! what's the 50% of "x" anyway?
[tex]\begin{array}{|c|ll} \cline{1-1} \textit{a\% of b}\\ \cline{1-1} \\ \left( \cfrac{a}{100} \right)\cdot b \\\\ \cline{1-1} \end{array}~\hspace{5em}\stackrel{\textit{50\% of x}}{\left( \cfrac{50}{100} \right)x}\implies 0.5x~\hfill {\LARGE \begin{array}{llll} g=0.5x - 8 \end{array}}[/tex]