Subtract polynomials: $ \, \color{blue}{ -4+8n^2-8n } \, $ , $ \, \color{orangered}{ \cancel{8n}-2n^2 -\cancel{8n} }$.

$$ \left( \color{blue}{-4+8n^2-8n} \right) - \left( \color{orangered}{ \cancel{8n}-2n^2 -\cancel{8n}} \right) = 10n^2-8n-4 $$

Step 1: First we have to write polynomials in descending order.

$$ \begin{aligned} P(x) &= 8n^2-8n-4 \\ Q(x) &= -2n^2 \\ \end{aligned} $$

Step 2: Write second polynomial below the first one. Making sure like terms are lined up.

$$ \begin{array}{lrrr} & 8n^2 & -8n & -4 \\ \color{red}{ - } & -2n^2 & & \\ \hline \end{array} $$

Step 3: Change ALL the signs in the second row.

$$ \begin{array}{lrrr} & 8n^2 & -8n & -4 \\ \color{red}{ - } & 2n^2 & & \\ \hline \end{array} $$

Step 4: Combine like terms (add each column):

$$ \begin{array}{lrrr} & 8n^2 & -8n & -4 \\ \color{red}{ - } & 2n^2 & & \\ \hline & 6n^2 & -8n & -4 \end{array} $$

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