You can put this solution on YOUR website!
Factor tree is listing all the factors of a number until you get to the prime numbers;
5 x 18
5 is a prime number, so it can not go any further, 18 has factors;
9 * 2
2 is prime so you stop there, but 9 has factors;
3 * 3
so the prime factorization is;
5 * 3 * 3 * 2
Hope you can understand this explanation, it is hard to explain without being able to actually show you.