Dead post, I know, but you can use image transformation tools like imwarp() to do the perspective correction.
A = imread('https://www.mathworks.com/matlabcentral/answers/uploaded_files/214874/56260471_568057093678369_4947302863245148160_n.jpg');
% these are the coordinates of the box corners
% you can get these using getpts() or impixelinfo() or datatips
boxm = [28 3; % [x y]
31 959;
1297 783;
1343 195];
% assert that this is where they're supposed to be
% any coordinates that define a rectangle
boxf = [30 30; % [x y]
30 900;
1300 900;
1300 30];
TF = fitgeotrans(boxm,boxf,'projective');
outview = imref2d(size(A));
B = imwarp(A,TF,'fillvalues',255,'outputview',outview);
imshow(B)
Normally, the easy way would be to just find the corners of the board, but since they aren't visible, just pick points that define a quadrilateral whose sides are apparently parallel to the board edges.
After the transformation, you may elect to crop the result and do further post-processing. Gotta love that glare, ghosting, and the feeble smudges of half-dry markers. Reminds me why I can't stand whiteboards.
B = imcrop(B,[14 33 1332 869]); % crop it
B = imflatfield(B,40); % try to suppress the glare
B = imadjust(B,[0.4 0.65]); % adjust levels
imshow(B)
You might be able to clean it up a bit further with some morphological operations. The compression artifacts are really going to end up being a limiting factor. That, and some of the written text is no darker than the ghost text.
B = imcomplement(imbothat(B,ones(7)));
imshow(B)
