Question 1133459
{{{x^2+16}}}
{{{x^2-(-16)}}}
{{{x^2-(-1)(4^2)}}}
{{{x^2-(4i)^2}}}
{{{(x+4i)(x-4i)}}}
If you allow imaginary numbers, then your given quadratic expression is factorable.



You can decide about your cubic expression on your own.  Try polynomial division with divisors of x+3 and x-3, and see what happens.