To follow up, by the complex conjugate roots theorem, we know the 4th root must be \(-i\), and so the quartic polynomial may be written as:
[MATH]f(x)=k(x+1)(x-2)(x-i)(x+i)=k(x^2-x-2)(x^2+1)=k\left(x^4-x^3-x^2-x-2\right)[/MATH]
Now, we can use the given point on the curve to determine the parameter \(k\):
[MATH]f(3)=k\left(3^4-3^3-3^2-3-2\right)=40k=80\implies k=2[/MATH]
And so we have:
[MATH]f(x)=2\left(x^4-x^3-x^2-x-2\right)=2x^4-2x^3-2x^2-2x-4[/MATH]