Consider $\left(A+B\right)\left(A-B\right)$. Multiplication can be transformed into difference of squares using the rule: $\left(a-b\right)\left(a+b\right)=a^{2}-b^{2}$.
$$A^{2}-B^{2}-\left(A+B\right)\left(A-B\right)$$
Consider $\left(A+B\right)\left(A-B\right)$. Multiplication can be transformed into difference of squares using the rule: $\left(a-b\right)\left(a+b\right)=a^{2}-b^{2}$.
$$A^{2}-B^{2}-\left(A^{2}-B^{2}\right)$$
To find the opposite of $A^{2}-B^{2}$, find the opposite of each term.
