2y=x-4, divide through by 2: y=x/2-2 so the slope is the x coefficient, ½. The slope of the perpendicular is -(1/½)=-2, so the equation of the perpendicular line has the form f(x)=-2x+a where a is a constant.
To find a plug in the point (8,-3): -3=-16+a, so a=13 and f(x)=-2x+13.