Since the integral equation is given (and assuming $X_0 = 0$), it suffices to show
$\displaystyle d(X_t^4 - 6tX_t^2 + 3t^2) = (4X_t^3 - 12tX_t)\, dX_t$.
To do this, set $f(t, x) = x^4 - 6tx^2 + 3t^2$ and apply Ito's lemma to get
$\displaystyle df(t, X_t) = \left(f_t + \frac{f_{xx}}{2}\right) dt + f_x \, dX_t$,
$\displaystyle df(t, X_t) = \left(-6X_t^2 + 6t + \frac{12X_t^2 - 12t}{2}\right) dt + (4X_t^3 - 12tX_t)\, dX_t$,
$\displaystyle df(t, X_t) = (-6X_t^2 + 6t + 6X_t^2 - 6t)\, dt + (4X_t^3 - 12tX_t)\, dX_t$,
$\displaystyle df(t, X_t) = 0\, dt + (4X_t^3 - 12tX_t)\, dX_t$,
$\displaystyle d(X_t^4 - 6tX_t^2 + 3t^2) = (4X_t^3 - 12tX_t)\, dX_t$.