If 2i is one zero of this function, then its conjugate, - 2i, is also zero of this function (this is because the function is a polynomial with real coefficients.)
This means, (x - 2i) and (x + 2i) are both factors of f(x).
`(x - 2i)(x+2i) = x^2+ 4`
Divide f(x) by `x^2 +4` to find the remaining factor:
See attached image for the long division problem that results in
x - 9.
Then, the last zero of the function is given by x - 9 = 0. It is 9.
The three zeroes of f(x) are 2i, - 2i and 9.