\starttext

\bTABLE
    \bTR
        \bTD a \eTD
        \bTD
            \startmathalignment[n=7,align={right,middle,right,middle,right,middle,right}]
            \NC  3x\NC +\NC 6y\NC -\NC 2z\NC =\NC -13\NR
            \NC -6x\NC -\NC 9y\NC +\NC 5z\NC =\NC  22\NR
            \NC  3x\NC +\NC 3y\NC -\NC 5z\NC =\NC -13\NR
            \stopmathalignment
        \eTD
    \eTR
    \bTR
        \bTD b \eTD
        \bTD
            \startmathalignment[n=7,align={right,middle,right,middle,right,middle,right}]
            \NC  x\NC +\NC 3y\NC +\NC 3z\NC =\NC -4\NR
            \NC  x\NC +\NC 4y\NC +\NC 5z\NC =\NC -5\NR
            \NC 2x\NC +\NC 4y\NC +\NC 3z\NC =\NC -5\NR
            \stopmathalignment
        \eTD
    \eTR
\eTABLE

\stoptext