This is a sum of cubes:
x⁶ - 27y³ = (x²)³ + (-3y)³
Recall that
a³ + b³ = (a + b) (a² - ab + b²)
So we have
x⁶ - 27y³ = (x² - 3y) ((x²)² - x² (-3y) + (-3y)²)
x⁶ - 27y³ = (x² - 3y) (x⁴ + 3x²y + 9y²)
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