function vectorfield
    clear;clf;
    A=[1,2;-2,1];
    [a,b]=solve('x+2*y=0','-2*x+y=0');
    a=double(a);
    b=double(b);
    x=a-2:0.2:a+2;
    y=b-2:0.2:b+2;
    [X,Y]=meshgrid(x,y);
    quiver(X,Y,A(1,1)*X+A(1,2)*Y,A(2,1)*X+A(2,2)*Y);

end

