You know that 2 parts ethane react with 7 parts oxygen .
Now you want to know how many parts ethane react with 21 parts oxygen.
Can you see that 21 is 7*3 ? Therefore if you multiply through by 3 you have your answer:
6 parts ethane react with 21 parts oxygen .
A more general equation:
Parts ethane = 21/7*2 = 6 parts ethane.