The second directional derivative of $f(x, y)$ is
$$D_{\mathfrak{u}}^{2} f(x, y)=D_{\mathrm{u}}\left[D_{\mathrm{u}} f(x, y)\right]$$
$$f(x, y)=x^{3}+5 x^{2} y+y^{3}$ and $\mathbf{u}=\left[\frac{3}{5}, \frac{4}{5}\right],calculate $D_{\mathrm{n}}^{2} f(2,1)$$