Answer
$a=-1$
Work Step by Step
Given: $a(a-3)=2a(a+3)-(a+3)(a-3)$
$a^2-3a=2a^2+6a-[a^2-3a+3a-9]$
$\implies a^2-3a=2a^2+6a-a^2+3a-3a+9$
$\implies a^2-3a=a^2+6a+9$
$\implies a^2+6a+9-a^2+3a=0$
or, $9a+9=0 \implies a=-\frac{9}{9}$
$\implies a=-1$
