Answer: 323, 437
Step-by-step explanation: Look for patterns! Here's what the factors of the current terms in the sequence give you:
6 = 2*3
15 = 3*5
35 = 5*7
77 = 7*11
143 = 11*13
221 = 13*17
So each term is actually the product of two consecutive prime numbers! At this rate, the next two terms are 17*19 = 323, and 19*23 = 437.