How should I calculate $||\underline{u}-\underline{w}||_{2}$? I'm trying to calculate $||\underline{u}-\underline{w}||_{2}$ where:
$$
u=\begin{bmatrix}1 & 3\\
2 & 2\\
3 & 1
\end{bmatrix},\,\,\, w=\begin{bmatrix}3 & 1\\
2 & 2\\
1 & 3
\end{bmatrix}
$$
I'm not familiar with the $|| \cdot ||_2$ operator and I'm not sure how to search it in the search engine. How should I calculate $||\underline{u}-\underline{w}||_{2}$?

What you're looking for is Matrix Norm $\|\cdot\|_p$ given by $$\|A\|_p:=\max_{|x|_p=1}|Ax|_p$$ where $|x|_p$ is the vector norm. In particular for $p=2,$ $$\|A\|_2=\sqrt{\lambda_{\max}(A^*A)}$$ where $\lambda_{\max}(A)$ denotes the largest eigenvalue of $A.$ 
