Show that the integral $\int(xu - 3tu^2) dx$ is invariant for the $KdV$ equation.

Question

I have a problem solving the problem about showing the integral is invariant for the KdV equation. The problem is as following:

Show that integral of $\int(xu - 3tu^2) dx$ is invariant for the $KdV$ equation.

Can anyone help? I'm kind of stuck here...