jmc

Recall the sum of cubes factorization $a^3+b^3=(a+b)(a^2-ab+b^2).$ We plug in the numbers from the equations given to obtain $81=(3)(a^2-ab+b^2)$. Therefore, $a^2-ab+b^2=27$. We also know that $(a+b{)}^2=9=a^2+2ab+b^2$. We use the two equations $$a^2+2ab+b^2=9$$and $$a^2-ab+b^2=27.$$By subtracting the second equation from the first, we get that $2ab+ab=9-27$. Therefore, $3ab=-18$, so $ab=\boxed{-6}$.