How to create matrices

Generally the easiest way is:

$
  L = \left[
    \begin{array}{cc}
       x & y \\
       z & w
    \end{array} \right]
$

A good reference about the topic can be found here:

http://www.rizauddin.com/2009/05/how-to-create-a-matrix-in-latex/

An even easier way to do this is to use pmatrix as

$
  \begin{pmatrix}
     x & y \\
     z &  w
  \end{pmatrix}
$

 However this latter will always put the matrix inside round  parenthesis.