What is the coefficient of the leading term in that binomial?

15 is the coefficient

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.