begin{equation}
begin{bmatrix}
  X_1 \
  X_2 \
end{bmatrix}
= - frac{1}{xi_{11},xi_{22} - xi_{21},xi_{12}}
begin{bmatrix}
   ,xi_{11} & phantom{-}xi_{12} \
   phantom{-}xi_{21} & ,xi_{22} \
end{bmatrix}
begin{bmatrix}
  xi_{10} \
  xi_{20} \
end{bmatrix}
end{equation}