You have the following equation:
2x - y = 4
Solve the previous equation for y:
y = 2x - 4
The slope of the previous line is m = 2.
Consider that the slope of a perpendicular line related to the previous one:
m' = -1/m
then, the slope of the perpendicular line:
m = -1/2
From the given answer choices you can notice that the following equation is the perpendicular line:
y = -1/2 x + 12