Explicación paso a paso:
a. (3a³-8a² +12c-9) (7a²) = 21a⁵ - 56a⁴ + 84a²c - 63a²
b. (6mn + 10) (7a - 2b) = 42amn + 70a - 12bmn - 20b
c. (5x + 8y + 12) (-2xy + 3) = -10x²y - 16xy² - 24xy +15x + 24y + 36
d. (-3x³y - 2x²y² - xy³) (x² + y²) = -3x⁵y -2x⁴y² - x³y³ -3x³y³ - 2x²y⁴ - xy⁵