A = [1 1 1; 2 1 3; 3 4 -2];
B = [4; 7; 9];
AB = [A, B];

alpha = AB(2, 1) / AB(1, 1);
AB(2, :) = AB(2, :) - (alpha * AB(1, :));
alpha = AB(3, 1) / AB(1, 1);
AB(3, :) = AB(3, :) - (alpha * AB(1, :));

alpha = AB(3, 2) / AB(2, 2);
AB(3, :) = AB(3, :) - (alpha * AB(2, :));

x = zeros(3, 1);
x(3) = AB(3, end) / AB(3, 3);
x(2) = (AB(2, end) - AB(2, 3) * x(3)) / AB(2, 2);
x(1) = (AB(1, end) - AB(1, 3) * x(3) - AB(1, 2) * x(2)) / AB(1, 1);

disp(x);