Lens formula: 1/v - 1/u = 1/f
u = -12 cm
v = 24 cm
1/f = (1/24) - 1/(-12)
= 1/24 + 1/12
f = 8 cm.
When u = -12 cm, the object is placed between F and 2F and the image is formed beyond 2F on the other side. This is a enlarged inverted image. If the object is moved away from the lens, the image moves towards the lens. So the screen should be moved towards the lens.
Magnification m = v/u = 24/(-12) = -2
If the denominator increases, v decreases and hence the magnification decreases. So the image is smaller than the previous image.