Given data:
The first root of the polynomial is x=2.
The second root of the polynomial is x=-4i.
The imaginary roots are always in pair, the third root of the polynomial is,
x=+4i.
The expression for the polynomial is,
f(x)=(x-2)(x+4i)(x-4i)
=(x-2)(x^2-16i^2)
=(x-2)(x^2+16)
=x^3+16x-2x^2-32.
Thus, the polynoial of least degree is x^3-2x^2+16x-32.