factor the polynomial 4y4 - 12x2 y2 + 9x4
This is a perfect square trinomial: 4y^4 = (2y²)² and 9x^4 = (3x²)²
Since the last sign is a '+' and the first term is a '-', it will factor into two factors, each one containing a minus sign.
Try (2y² - 3x²)(2y² - 3x²)