we know that
if two lines are perpendicular
then
m1*m2=-1
where
m1 and m2 are the slopes of the lines
step 1
For Y=(1/2)X-1
slope m1=(1/2)
so
m2=-1/m1-----------> m2=-1/(1/2)-------> m2=-2
step 2
with m=-2 and the point (2,5) find the equation of the line
y-y1=m*(x-x1)------> y-5=-2*(x-2)-----> y=-2x+4+5------> y=-2x+9
the answer is
y=-2x+9
see the attached figure