Multiply polynomial $ \, \color{blue}{ 3x-9 } \, $ by $ \, \color{orangered}{ 3x+9 }$.

$$ \left( \color{blue}{3x-9} \right) \cdot \left( \color{orangered}{3x+9} \right) = 9x^2-81 $$

In this example we are multiplying two binomials so FOIL method can be used.

$$ \begin{aligned} \left( \color{blue}{ 3x-9}\right) \cdot \left( \color{orangered}{ 3x+9}\right) &= \underbrace{ \color{blue}{3x} \cdot \color{orangered}{3x} }_{\text{FIRST}} + \underbrace{ \color{blue}{3x} \cdot \color{orangered}{9} }_{\text{OUTER}} + \underbrace{ \left( \color{blue}{-9} \right) \cdot \color{orangered}{3x} }_{\text{INNER}} + \underbrace{ \left( \color{blue}{-9} \right) \cdot \color{orangered}{9} }_{\text{LAST}} = \\ &= 9x^2 + 27x + \left( -27x\right) + \left( -81\right) = \\ &= 9x^2 + 27x + \left( -27x\right) + \left( -81\right) = \\ &= 9x^2-81; \end{aligned} $$

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