Question 1198743
{{{a(x+1)(x-(2+i))(x-(2-i))=-15}}}
{{{a(x+1)(x-2-i)(x-2+i)=-15}}}
{{{a(x+1)((x-2)^2-1)=-15}}}
{{{a(x+1)(x^2-4x+4+1)=-15}}}
{{{a(x+1)(x^2-4x+5)=-15}}}
The multiplication for the polynomial is not yet finished, but letting x=0 allows to find the factor, "a".