\[2(a - 3)^{2} - 2a^{2} =\]
\[= 2\left( a^{2} - 6a + 9 \right) - 2a^{2} =\]
\[= 2a^{2} - 12a + 18 - 2a^{2} =\]
\[= - 12a + 18.\]