$a\text{)}2mn+6m+3n+9=2m\left(n+3\right)+3\left(n+3\right)=\left(n+3\right)\left(2m+3\right)$    


$b\text{)}3p^2-3q+9p-pq=3p^2+9p-3q-pq=3p\left(p+3\right)-q\left(3+p\right)=\left(p+3\right)\left(3p-q\right)$