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sqdancefan
"Factor by grouping" generally means you divide the 4 terms into groups of 2 terms.
.. (5x^2 +40x) -(x +8)
Now, you factor each group.
.. 5x(x +8) -1(x +8)
Now, you factor out the common factor, x+8.
.. (5x -1)(x +8)