When you multiply two prime numbers you always get answer with prime number?

No. If you multiply any numbers, those numbers are factors of whatever product you get. Therefore, this product is a composite number; it has atleast the two factors you multiplyed before. Compposite numbers are never prime.

If the two factors above were prime, you would still end up with a composite number.

For example: 3 times 5 equals 15. The factors of 15 are 1, 3, 5, and 15. 15 is composite.

7 time 51 equals 357. The factors of 357 are 1, 7, 51, and 357. 357 is composite.