(1/4y³ - 5/6)²

(a - b)² = a² - 2ab + b²

(1/4y³)² - 2 * (1/4y³) * 5/6 + (5/6)²

(1/4y³)² = 1/6y⁶

2 * (1/4y³) * 5/6 = (1/4y³) * 5/3 = 5/12y³

(5/6)² = 25/36

1/6y⁶ - 5/12y³ + 25/36