It would be 3. This is because just by looking at the first term of the quadratic we need to make 3x^2 with our two factors. The leading coefficient is 3 in the quadratic, thus we must consider its factors in the two binomials making the factors of the quadratic. 3’s factors are just 1 and 3. Since, one is already taken by the binomial given(the imaginary one in front of the x) we can safely assume the second factor to have a leading coefficient of 3.