Упростить: (a+3)^2-(a+3)(2a-3).

\[(a + 3)^{2} - (a + 3)(2a - 3) =\]

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

\[= a^{2} + 6a + 9 - 2a^{2} - 3a + 9 =\]

\[= - a^{2} + 3a + 18.\]