8. (2a+6b)(6b-2a)-(2a+6b)²

$$\begin{aligned}&(2a+6b)(6b-2a)-(2a+6b)^2 \\&= (36b^2 - 4a^2) - (4a^2 + 24ab + 36b^2) \\&= 36b^2 - 4a^2 - 4a^2 - 24ab - 36b^2 \\&= -8a^2 - 24ab\end{aligned}$$ -8a² - 24ab