Divide using synthetic division $$ ( 3x^{3}+17x^{2}+21x-9 ) \div ( x+3 ) $$
Answer
The synthetic division table is:
$$ \begin{array}{c|rrrr}-3&3&17&21&-9\\& & -9& -24& \color{black}{9} \\ \hline &\color{blue}{3}&\color{blue}{8}&\color{blue}{-3}&\color{orangered}{0} \end{array} $$The solution is:
$$ \frac{ 3x^{3}+17x^{2}+21x-9 }{ x+3 } = \color{blue}{3x^{2}+8x-3} $$Explanation
Step 1 : Write down the coefficients of the dividend into division table. Put the zero from $ x + 3 = 0 $ ( $ x = \color{blue}{ -3 } $ ) at the left.
$$ \begin{array}{c|rrrr}\color{blue}{-3}&3&17&21&-9\\& & & & \\ \hline &&&& \end{array} $$Step 1 : Bring down the leading coefficient to the bottom row.
$$ \begin{array}{c|rrrr}-3&\color{orangered}{ 3 }&17&21&-9\\& & & & \\ \hline &\color{orangered}{3}&&& \end{array} $$Step 2 : Multiply by the number on the left, and carry the result into the next column: $ \color{blue}{ -3 } \cdot \color{blue}{ 3 } = \color{blue}{ -9 } $.
$$ \begin{array}{c|rrrr}\color{blue}{-3}&3&17&21&-9\\& & \color{blue}{-9} & & \\ \hline &\color{blue}{3}&&& \end{array} $$Step 3 : Add down last column: $ \color{orangered}{ 17 } + \color{orangered}{ \left( -9 \right) } = \color{orangered}{ 8 } $
$$ \begin{array}{c|rrrr}-3&3&\color{orangered}{ 17 }&21&-9\\& & \color{orangered}{-9} & & \\ \hline &3&\color{orangered}{8}&& \end{array} $$Step 4 : Multiply by the number on the left, and carry the result into the next column: $ \color{blue}{ -3 } \cdot \color{blue}{ 8 } = \color{blue}{ -24 } $.
$$ \begin{array}{c|rrrr}\color{blue}{-3}&3&17&21&-9\\& & -9& \color{blue}{-24} & \\ \hline &3&\color{blue}{8}&& \end{array} $$Step 5 : Add down last column: $ \color{orangered}{ 21 } + \color{orangered}{ \left( -24 \right) } = \color{orangered}{ -3 } $
$$ \begin{array}{c|rrrr}-3&3&17&\color{orangered}{ 21 }&-9\\& & -9& \color{orangered}{-24} & \\ \hline &3&8&\color{orangered}{-3}& \end{array} $$Step 6 : Multiply by the number on the left, and carry the result into the next column: $ \color{blue}{ -3 } \cdot \color{blue}{ \left( -3 \right) } = \color{blue}{ 9 } $.
$$ \begin{array}{c|rrrr}\color{blue}{-3}&3&17&21&-9\\& & -9& -24& \color{blue}{9} \\ \hline &3&8&\color{blue}{-3}& \end{array} $$Step 7 : Add down last column: $ \color{orangered}{ -9 } + \color{orangered}{ 9 } = \color{orangered}{ 0 } $
$$ \begin{array}{c|rrrr}-3&3&17&21&\color{orangered}{ -9 }\\& & -9& -24& \color{orangered}{9} \\ \hline &\color{blue}{3}&\color{blue}{8}&\color{blue}{-3}&\color{orangered}{0} \end{array} $$Bottom line represents the quotient $ \color{blue}{ 3x^{2}+8x-3 } $ with a remainder of $ \color{red}{ 0 } $.