What is this answer?

Question

For the following​ polynomial, one zero is given. Find the remaining zeros.

​P(x)=x4+27x2−324​,

6i is a zero.

The remaining zeros are ?

​(Simplify your answer. Use a comma to separate answers as​ needed.)

Answer 1

x^4 -27x^2 - 324 = 0

x=6i is one zero

imaginary zeroes come in conjugate pairs

so x=-6i is a 2nd zero

(x-6i)(x+6i) = x^2+36 = 0

Let z =x^2

z+36 = 0

z^2 +27z -324 = 0

(z+36)(x-9) = 0

z=-36, z=9

x =sqrz = -6i, 6i, 3 and -3

if you couldn't factor the z equation, then complete the square or use the quadratic formula

or divide it by z+36 to get z-9 use long or synthetic division

then x = + and -sqr9

zeroes are square roots of 9 and -36

= 3, -3, 6i and -6i