Find all values of the real number a so that the four complex roots of
\(z^4 - 6z^3 + 11az^2 - 3(2a^2 + 3a - 3) z + 1 = 0\)
form the vertices of a parallelogram in the complex plane. Enter all the values, separated by commas.
The possible values of a are -4, 1, and 2.
