How to transform to grayscale image?

I want to transform to grayscale image but I should not by the matlab commands I must use only matrix operations.How can I do? e.g=(262x262 x3 uint8 image).

note:ı can use basic commands sometimes if they requires.

Answer 1

[KSSV]

I = imread(image) ;
R = I(:,:,1) ; 
G = I(:,:,2) ; 
B = I(:,:,3) ; 
I = 1/3*(R+G+B) ;
I = uint8(I) ;
imshow(I)