Seperating in Digital Image Processing

I coded this question but it don't work. I have a error. If someone help me, i will be very happy. Thans.

My tried code block:

img=imread('peppers.png');

img=(im2double(img));

R(img)=img(:,:,1); %we chosed channal 1

G=img(:,:,2);

B=img(:,:,3);

x=16; y=16;

m=2048; n=1024;

sub_image=R(img)(x:x+m,y:y+n);

sub_image=R(img)(16:2048, 16:1024);

imshow(sub_image);

imwrite(sub_image,'sub_region.jpg');