Iam terrible at math ,so do consider the try

We know that
if two lines are perpendicular
so
m1*m2=-1

step 1
the line  
y=(-1/2x)-1-----> has a slope m1=-1/2
so
m1*m2=-1
m2=2

step 2
with m2=2 and the point (6,9) find the equation of the line
y-y1=m*(x-x1)----> y-9=2*(x-6)---> y=2x-12+9----> y=2x-3

the answer is
y=2x-3

see the attached figure