Divide using synthetic division $$ ( x^{3}+6x^{2}-9x-8 ) \div ( x+7 ) $$
Answer
The synthetic division table is:
$$ \begin{array}{c|rrrr}-7&1&6&-9&-8\\& & -7& 7& \color{black}{14} \\ \hline &\color{blue}{1}&\color{blue}{-1}&\color{blue}{-2}&\color{orangered}{6} \end{array} $$The solution is:
$$ \frac{ x^{3}+6x^{2}-9x-8 }{ x+7 } = \color{blue}{x^{2}-x-2} ~+~ \frac{ \color{red}{ 6 } }{ x+7 } $$Explanation
Step 1 : Write down the coefficients of the dividend into division table. Put the zero from $ x + 7 = 0 $ ( $ x = \color{blue}{ -7 } $ ) at the left.
$$ \begin{array}{c|rrrr}\color{blue}{-7}&1&6&-9&-8\\& & & & \\ \hline &&&& \end{array} $$Step 1 : Bring down the leading coefficient to the bottom row.
$$ \begin{array}{c|rrrr}-7&\color{orangered}{ 1 }&6&-9&-8\\& & & & \\ \hline &\color{orangered}{1}&&& \end{array} $$Step 2 : Multiply by the number on the left, and carry the result into the next column: $ \color{blue}{ -7 } \cdot \color{blue}{ 1 } = \color{blue}{ -7 } $.
$$ \begin{array}{c|rrrr}\color{blue}{-7}&1&6&-9&-8\\& & \color{blue}{-7} & & \\ \hline &\color{blue}{1}&&& \end{array} $$Step 3 : Add down last column: $ \color{orangered}{ 6 } + \color{orangered}{ \left( -7 \right) } = \color{orangered}{ -1 } $
$$ \begin{array}{c|rrrr}-7&1&\color{orangered}{ 6 }&-9&-8\\& & \color{orangered}{-7} & & \\ \hline &1&\color{orangered}{-1}&& \end{array} $$Step 4 : Multiply by the number on the left, and carry the result into the next column: $ \color{blue}{ -7 } \cdot \color{blue}{ \left( -1 \right) } = \color{blue}{ 7 } $.
$$ \begin{array}{c|rrrr}\color{blue}{-7}&1&6&-9&-8\\& & -7& \color{blue}{7} & \\ \hline &1&\color{blue}{-1}&& \end{array} $$Step 5 : Add down last column: $ \color{orangered}{ -9 } + \color{orangered}{ 7 } = \color{orangered}{ -2 } $
$$ \begin{array}{c|rrrr}-7&1&6&\color{orangered}{ -9 }&-8\\& & -7& \color{orangered}{7} & \\ \hline &1&-1&\color{orangered}{-2}& \end{array} $$Step 6 : Multiply by the number on the left, and carry the result into the next column: $ \color{blue}{ -7 } \cdot \color{blue}{ \left( -2 \right) } = \color{blue}{ 14 } $.
$$ \begin{array}{c|rrrr}\color{blue}{-7}&1&6&-9&-8\\& & -7& 7& \color{blue}{14} \\ \hline &1&-1&\color{blue}{-2}& \end{array} $$Step 7 : Add down last column: $ \color{orangered}{ -8 } + \color{orangered}{ 14 } = \color{orangered}{ 6 } $
$$ \begin{array}{c|rrrr}-7&1&6&-9&\color{orangered}{ -8 }\\& & -7& 7& \color{orangered}{14} \\ \hline &\color{blue}{1}&\color{blue}{-1}&\color{blue}{-2}&\color{orangered}{6} \end{array} $$Bottom line represents the quotient $ \color{blue}{ x^{2}-x-2 } $ with a remainder of $ \color{red}{ 6 } $.