Since coefficient of x^2 is prime and both second and third term are positive, it must be in form (3x+_)(x+_).
There is three possibles for these blank spaces;
1,4
2,2
4,1
Because coefficient of second term is 13 = 1+12 = 1+3*4, these blank space should be 1,4
So your answer is (3x+1)(x+4)
Hope this makes sense.
