This is already in prime factored form. If you multiply 2, 3, 5, and 7 you get 2*3*5*7 = 6*35 = 210
So if your book asked "what is the prime factorization of 210?" then the answer would be 2*3*5*7
I'm using the asterisk symbol * in place of x because x is often used as a variable in algebra.
