Yes, that is a bad question. It's ambiguous - it says that a sliding 3x3 window is used but then it also says a window that is (2*L+1) by (2*L+1) is used. That can't be unless L = exactly 1. It's contradictory for any other value of L. In general, assign your L, assign your kernel, then call conv2()
L = 1; % or any odd number.
windowWidth = 2 * L + 1;
kernel = ones(windowWidth)/windowWidth^2;
iBar = conv2(double(grayImage), kernel, 'same');
TM = abs(double(grayImage) - iBar);
imshow(TM, []);
