Before coming to ImageMagick visit Poor Person's Image Mapper and find the top left and lower right coordinates of the area you want to cut out.

Next you'll need to know the size of the area you want to cut out. Take the coordinates you got at the Image Mapper and subtract the upper left coordinates from the lower right coordinates. (lower width - upper width) & (lower height - upper height)
Width of cutHeight of cut
 318
-138
=180
 270
-  65
=205

These coordinates will be used to cut out your image.
Enter the coordinates as follows:

Parameter: