Two lines are perpendicular if the product of the slopes equals -1, or m1m2 = -1
In other words, m2 = - 1 / m1
So the slope of the perpendicular line we're looking must have a slope of - 1 /(1/2) = -2
What do we have, so far, for the equation of the perpendicular line, in standard form?
y = -2x + b
How do we find b? Simply by plugging the coordinates of point (1, -1) into the above equation. After all, the perpendicular line must go thru that point.
-1 = -2(1) + b
Solve for b, you should be able to take it from here.