Set the argument -1/2x equal to the x coordinate of the given point 6. Solve for x
-1/2x = 6 x = 6(-2) x = -12
So x = -12 makes the equation -1/2x = 6 true.
If we plug x = -12 into f(-1/2x) we get f(6)
Since we know that (6,-8) is on the graph of f(x), we know that (-12,-8) is on the graph of f(-1/2x)
What happens is that there's a reflection over the y axis (due to the minus sign in -1/2x) and there's a horizontal stretch by a factor of 2. So the graph of f(-1/2x) appears to be wider than f(x).
