jmc

Simplifying, we get $x^2-6x-27=0$. Now we factor and get $(x-9)(x+3)=0$, hence we have $x=9$ and $x=-3$. Since $a\ge b$, $a=9$ and $b=-3$, so $2a-3b=2(9)-3(-3)=18+9=\boxed{27}$.