\left| \left| \matrix{ a & b \cr c & d \cr e & f \cr} \right| \right| \; \left| \left| \matrix{ x & y \cr z & u \cr} \right| \right| \; = 
\; \left| \left| \matrix{ ax + bz & ay + bu \cr cx + dz & cy + du \cr ex + fz & ey + fu \cr} \right| \right|