Multiply polynomial $ \, \color{blue}{ 9d+4 } \, $ by $ \, \color{orangered}{ 9d-4 }$.

$$ \left( \color{blue}{9d+4} \right) \cdot \left( \color{orangered}{9d-4} \right) = 81d^2-16 $$

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

$$ \begin{aligned} \left( \color{blue}{ 9d+4}\right) \cdot \left( \color{orangered}{ 9d-4}\right) &= \underbrace{ \color{blue}{9d} \cdot \color{orangered}{9d} }_{\text{FIRST}} + \underbrace{ \color{blue}{9d} \cdot \left( \color{orangered}{-4} \right) }_{\text{OUTER}} + \underbrace{ \color{blue}{4} \cdot \color{orangered}{9d} }_{\text{INNER}} + \underbrace{ \color{blue}{4} \cdot \left( \color{orangered}{-4} \right) }_{\text{LAST}} = \\ &= 81d^2 + \left( -36d\right) + 36d + \left( -16\right) = \\ &= 81d^2 + \left( -36d\right) + 36d + \left( -16\right) = \\ &= 81d^2-16; \end{aligned} $$

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