First, we will expand the bracket:
(2-3i)^2 = 4 - 2(2)(3i) + 9i^2
= 4 - 12i - 9 = -5 - 12i
now, we know that c + di = -5 - 12i
This means that the real part is equal to the real part and the imaginary part is equal to the imaginary part.
Based on this, c = -5