6s^2 +40s -64
= 2(s +8)(3s -4)
Choose one of (s +8) or (3s -4), whichever is on your list.
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After you factor out the 2, you have
.. 3s^2 +20s -32
You want to find two factors of 3*(-32) = -96 that add up to +20.
.. -96 = -1*96 = -2*48 = -3*32 = -4*24 = -6*16 = -8*12
These factor pairs total to 46, 29, 20, 10, 4, so the factor pair (-4, 24) is the one you're looking for.
Now, you can rewrite the trinomial as
.. 3s^2 -4s +24s -32 . . . . . . splitting the middle term into 2 parts that add to 20s
and factor by grouping.
.. s(3s -4) +8(3s -4)
.. (s +8)(3s -4)