Solution: Apply the formula \( (a - b)^2 = a^2 - 2ab + b^2 \). Here, \( (2x)^2 - 2 \cdot 2x \cdot 3y + (3y)^2 = 4x^2 - 12xy + 9y^2 \). oxed{4x^2 - 12xy + 9y^2}