Antonia is factoring the polynomial, which has four terms.21x3−63x2+15x−453[7x3−21x2+5x−15]3[7x2(x−3)+5(x−3)]

After factoring out 3, group the remaining terms: 7x³ − 21x² + 5x − 15 = 7x²(x − 3) + 5(x − 3). Both groups contain the factor (x − 3), so factoring it out gives 3(7x² + 5)(x − 3). This matches choice C.

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