Recordem

$$\begin{gathered} (a+b{)}^2=a^2+2ab+b^2 \\ (a-b{)}^2=a^2-2ab+b^2 \\ (a+b)·(a-b)=a^2-b^2 \end{gathered}$$    

Exemples:

$$\begin{gathered} (x+3{)}^2=x^2+2·x·3+3^2=x^2+6x+9 \\ (x-3{)}^2=x^2-2·x·3+3^2=x^2-6x+9 \\ (x+3)·(x-3)=x^2-3^2=x^2-9 \end{gathered}$$