Question 964699
Real coefficients means complex roots occur in complex conjugate pairs.
So if {{{3i}}} is a root, so is {{{-3i}}}.
{{{f(x)=(x-4)(x+4)(x-3i)(x+3i)}}}
{{{f(x)=(x^2-16)(x^2+9)}}}
{{{f(x)=x^4+9x^2-16x^2-144}}}
{{{f(x)=x^4-7x^2-144}}}