Factor 3x^2 + x

only possible choices at the front
(3x .....)(x ......)

for each of those, choices at the back
(..... +4)(.....-1)
(..... -4)(..... +1)
(..... +2)(..... -2)
(...... -2)(..... +2)
try them

You should really learn one or more of the standard ways to factor trinomials.
Surely one has been taught to you in class.

here is a quick way to check
1. multiply the first and last coefficients ----- (3)(-4) = -12
2. look at the coefficient of the middle term ----- +1
3. what are factors of -12 that have a sum of +1 ???
I would "guess" at +4 and -3
(+4) + (-3) = 1
(4)(-3) = -12

So it does factor!

(3x +4)(x-1)

3 and 4 differ by one and you need one :)

Thank you