Antonia is factoring the polynomial, which has four terms. 21x3−63x2 15x−45 3[7x3−21x2 5x−15] 3[7x2(x−3) 5(x−3)] Which is the completely factored form of her polynomial? 21x2(x − 3) 36x2(x − 3) 3(7x2 5) (x − 3) 3(12x2 5) (x − 3).