If 2i is a zero then so is -2i, together they make the real factor (x-2i)(x+2i)=x2+4.
The other two zeroes create the factors (x-3)(x-2)=x2-5x+6. The polynomial has the form:
f(x)=a(x2-5x+6)(x2+4)=
a(x4+4x2-5x3-20x+6x2+24)=a(x4-5x3+10x2-20x+24).
f(1)=-15, so -15=a(1-5+10-20+24)=10a, so a=-1.5 or -3/2.
f(x)=-3x4/2+15x3/2-15x2+30x-36.