The triangle shown is dilated with scale factor =1/2 which is the image of vertex s after the dilation

The dilation factor is 1/2. This means the image would be half to the pre-image.
The coordinates for S are (2,4). So, its image is
[tex](\frac{1}{2}\cdot2,\frac{1}{2}\cdot4)=(1,2)[/tex]