\[a^{2}(a - 2) - a(a - 2)^{2} = (a - 2) \cdot\]
\[\cdot \left( a^{2} - a(a - 2) \right) = (a - 2) \cdot\]
\[\cdot \left( a^{2} - a^{2} + 2a \right) = 2a(a - 2)\]