Section 14.6: Directional Derivatives & Gradient Vector

The second directional derivative of \(f(x, y)\) is \(D^2_{\mathbf{u}}f(x, y) = D_{\mathbf{u}}[D_{\mathbf{u}}f(x, y)]\) If \(f(x, y) = x^3 + 5x^2y + y^3\) and \(\mathbf{u} = \langle \frac{3}{5}, \frac{4}{5} \rangle\), calculate \(D^2_{\mathbf{u}}f(2, 1)\).