Hey there! I'm happy to help!
A prime number is a number whose only factors are one and itself. 5 is a prime number because the only whole numbers you can multiply to get 5 are 1 and 5.
Prime factorization is finding the prime numbers that multiply to make that number.
To do this, you see what numbers it divides by. 15 is divisible by 1, giving us 15. It is not divisible by 2, but it is divisible by 3, which gives us 5.
Now, we see if 3 and 5 can be factorized even more, but they cannot as they are both prime numbers. Therefore, the prime factorization of 15 is 3,5.
Have a wonderful day! :D