Multiply These Trinomials, and Simplify $\left(a^{2}+3 a b-9 b\right)\left(a-3 a b-b^{2}\right)$

Essay

Multiply these trinomials, and simplify: $\left(a^{2}+3 a b-9 b\right)\left(a-3 a b-b^{2}\right)$