Let \( f:[1,\infty)\to \)(\(\mathbb{R}\)) be a differentiable function. If \( 6\int_{1}^{x} f(t)\,dt=3x f(x)+x^{3}-4 \) for all \( x\ge 1 \), then the value of \( f(2)-f(3) \) is:
\( 3 \)
\( -4 \)
\( -3 \)
\( 4 \)