A positive integer has exactly eight positive divisors. If two of the divisors are 21 and 35, what is the number?

Well, just so you understand what you have with what you are given:

You have 2 of the 8 positive divisors and you need 6 more.

You also have the number, which is  21 x 35 = (the number).

Would the factors of 21 and 35 give you any more information?

(The answer is "yes"---and that's plenty of help.)