Answer
$240$
Work Step by Step
Residuals and Sum-of-Squares Error (SSE)
If we model a collection of data $(x_{1}, y_{1})$ , . , $(x_{n}, y_{n})$ with a linear equation $\hat{y}=mx+b$,
then the residuals are the $n$ quantities (Observed Value-Predicted Value):
$(y_{1}-\hat{y}_{1}), (y_{2}-\hat{y}_{2}), \ldots, (y_{n}-\hat{y}_{n})$ .
The sum-of-squares error (SSE) is the sum of the squares of the residuals:
SSE $=(y_{1}-\hat{y}_{1})^{2}+(y_{2}-\hat{y}_{2})^{2}+\cdots+(y_{n}-\hat{y}_{n})^{2}.$
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Build a table, column by column
\begin{array}{cc|c|c|c|cc}
& x & y & y'=2x-8 & (y-y') & (y-y')^2 \\
\hline & 2 & 4 & -4 & 8 & 64 \\
& 6 & 8 & 4 & 4 & 16 \\
& 8 & 12 & 8 & 4 & 16 \\
& 10 & 0 & 12 & -12 & 144 \\
\hline & & & & {\bf SSE}= & {\bf 240} \\\hline
\end{array}
