We know we have to have a +1 and a -1 in the factored form because of -1 is the last number in the polynomial.
If we take out an (x+1)(?-1)
What times x is going to give us 3x^2?
(x+1)(3x-1)
If we distrubute or foil this we get the polynomial back
x times 3x = 3x^2
x times -1 = -1x
1 times 3x = 3x
1 times -1 = -1
3x^2+2x-1
The answer checks out.b