A prime factor is a factor that cannot be factored further.
Starting with 150, determine some of the factors: 150 = 15(10)
Continue: 3 5 2 5 = {2, 3, 5} None of these can be factored, so the prime factors of 150 are {2, 3, 5}. Of course, you could include 1 as a prime factor, in which case the prime factor list would be {1, 2, 3, 5}.