this code is not working it defied my debug plis assist...

x0=[1 4];
tspan=[0 10];
[t,x]=ode45('Assignment_6a',tspan,x0);
% figure(1)
% plot(t,x(:,1),'r-',t,x(:,2),'k--');
% xlabel('time, sec');
% ylabel('x1(t) and x2(t)');
% title('Time response');
% hold on
figure(A)
plot(x(:,1),x(:,2));
xlabel('x1');
ylabel('x2');
title('phase portrait');
legend('phase portrait trajectory');

hold on

here is its dynamics.....

function xdot=Assignment_6a(t,x)

a=1;

k=1;

% Defining the sliding mode controller
s=a*x(1)+x(2);

u=10*(-a*x(2)-sign(s)*(2*(x(1)^2+2*abs(x(2)))-k*s));

% Now the system equations
xdot(1)=x(2);
xdot(2)=1.3*x(1)^2+1.5*abs(x(2))+(1.1+sin(x(1)+x(2)))*u;
xdot=xdot';
1;