You can ignore point2 (c.x2,x.y2). Jcrop is just being helpful by giving you a rectangles size (c.w,c.h), starting point (c.x,c.y), and ending point (c.x2,c.y2).
ImageMagick has it's own Image Geometry format, so you just need the size & offset (starting point).
function showCoords(c)
{
var IMGeometry = String.concat(c.w,"x",c.y,"+",c.x,"+",c.y);
//=> "175x125+50+50"
// Assuming you are using node-imagemagick
im.convert(["source.jpg","-crop",IMGeometry,"out.jpg"],function(err, stdout){
// Callback
});
}