(2m - 3n)^2 = (2m - 3n)(2m - 3n)
Expanding:
= 4m^2 - 6mn - 6mn + 9n^2
= 4m^2 - 12mn + 9n^2

Therefore, (2m - 3n)^2 simplifies to 4m^2 - 12mn + 9n^2.