This is not a valid expression to initialize a cell in Matlab:
cell = ['b(1,1)+b(1,2)';'b(2,1)+b(2 ,3)';'b(3,3)+b(3,2)'];
You have to use the curly brackets { }:
cell = {'b(1,1)+b(1,2)';'b(2,1)+b(2 ,3)';'b(3,3)+b(3,2)'};
BTW, if you want to solve linear equation of the form AX+b=0, you can simply try X=-inv(A)*b
% Define system
A = [2 3 1; 7 -1 1; 4 0 5];
b = [1 0 1].';
% Solve system
X = -inv(A)*b;