Hi Juan,
First of all, when one of zeros is complex number you can be sure THE CONJUGATE OF THAT IS ALSO OTHER ZERO, so all zeros are
x =2 x = -2 x = 3i x= -3i
f(x) = (x - 2) (x + 2) ( x - 3i) (x + 3i)
f(x) = ( x^2 - 4) ( x^2 -9i^2)
f(x) = ( x^2 - 4) ( x^2 +9)
f(x) = x^4 + 5 x^2 -36
I hope it is useful,
Minoo