\(control[u_List] := 
	
      Module[{aa, x1, x2, lu, xx, maxx1, maxx2, minx1, minx2}, 
		
        aa = {{2/3, \(-2\)/3}, {2/3, 2/3}}; 
		x1[0] = 0; x2[0] = 1; 
		
        x1[t_] := \((x1[t - 1] - x2[t - 1])\)/2 + 1/2\ u[\([t]\)]; 
        x2[t_] := \((x1[t - 1] + x2[t - 1])\)/2\  - \ u[\([t]\)]; 
		
        lu = Length[u]; 
		{x1[lu], x2[lu]}; 
	\ \ 
        xx = Table[{x1[i], x2[i]}, {i, 0, lu}]; 
			
        Show[Graphics[{Line[xx], 
              Text[{x1[lu], x2[lu]}, {x1[lu] - 1, x2[lu] + 1}], 
              Line[{{\(-5\), 0}, {5, 0}}], Line[{{0, \(-5\)}, {0, 5}}]}]]; 

        		\ x1[lu]\  + \ 3\ x2[lu]
			]\)

control[Table[i, {i, -6, 6}]]