What is one factor of 20x2 + 6x − 8

You can divide a factor of 2 from all terms:
  2(10x² +3x -4)

You can look for factors of 10×(-4) = -40 that add to 3.
  -40 = 40×-1 = 20×-2 = 10×-4 = 8×-5 . . . . . these last ones add to 3

Now, you know the factorization is
  2(10x +8)(10x -5)/10 = 2(5x +4)(2x -1)

One factor of 20x² +6x -8 is any of
  2
  5x +4
  2x -1
or any product of 2 of these.