As masters said, if a polynomial with real coefficients has a+ bi as zero it must also have it's complement, a- bi, as zero. A polynomial with real coefficients have 2, 31/2, 2i and 3i as zeros must also have -2i and -3i as zeros and so is of the form a(x- 2)(x- 31/2)(x- 2i)(x- 3i)(x+ 2i)(x+ 3i) and is degree 6.