By dividing your number by prime numbers until all the factors are prime.
210
105,2
35,3,2
7,5,3,2
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2*2*5*5
99 = 32 x 11
2 x 3 x 5
To find the prime factorization it helps to first divide by obvious divisors. In this case, the number ends in a 5, so we know it can divide by 5. Do this and you get 49. This is known from times tables to be 7x7. Thus the prime factorization of 245 is 5x7x7.
They are: 3*3 = 9 and 2*2*3 = 12