The missing number in the equation C4H10 13O2?

Hey so I am Answering this for Primavera Online High School . Which side note I have straight 'A's right know so if you are having trouble with school go to Primavera Online High School Or Middle School . It helps You and you only have two class a day. Thats great any ways the answer to this question is ....

2

C2H4 + (?)O2 => 2 CO2 + 2H2O

the answer should look like!!!

C2H4 + (2)O2 => 2 CO2 +2H2O

So In the short tha answer is two and i hope this helped you :) good luck!