REmember that
if two lines are perpendicular, then the product of their slopes is equal to -1
in this problem we have
f (x) = -1/2x +7
the slope is m=-1/2
so
the slope of the perpendicular line is
m=2
therefore
an equation perpendicular to the given line is
y=2x
i need a point to calculate the complete equation of the line