Identify the nature of each critical point:
\begin{align*}
f(0,0) &= 1040 \text{, saddle point} \\
f(0,4) &= 1296 \text{, local maximum} \\
f(0,-4) &= 1296 \text{, local maximum} \\
f(6,0) &= -256 \text{, local minimum} \\
f(6,4) &= 0 \text{, saddle point} \\
f(6,-4) &= 0 \text{, saddle point} \\
f(-6,0) &= -256 \text{, local minimum} \\
f(-6,4) &= 0 \text{, saddle point} \\
f(-6,-4) &= 0 \text{, saddle point}
\end{align*}