Solution

Knowing, that the focal length of a spherical Lens is $$\frac{R}{2}$$
We can use the formula:
$$\frac{1}{f} = \frac{1}{b} + \frac{1}{g}$$
to get the position of the Image. In this case $g = -R$ and $f = -\frac{R}{2}$(minus because it is a concave lens).Putting these values in the formula:
$$-\frac{2}{R} + \frac{1}{R} = -\frac{1}{R} = \frac{1}{b} \Leftrightarrow b = -R$$
This tells us that the image is at the exact same position as the object. The formula for the magnification is:
$$M = -\frac{b}{g}$$
in this case, $b = g$ so $M=-1$, that means, the image is just a flipped version of the object(mirrored along the x-axis)