ProofWiki:Sandbox

$$ \begin{array}{lcl} z       & = & a \\ f(x,y,z) & = & x + y + z \end{array} $$

$$ \begin{matrix} z       & = & a \\ f(x,y,z) & = & x + y + z \end{matrix} $$

$$ \begin{cases} 42 \\ 24 \end{cases} $$