Balance this equation. if a coefficient of "1" is required, choose "blank" for that box. c2h4 + o2 → co2 + h2o

When balancing  the  equation  you  need  to  know  how  many     moles  reacted  and  how  many were formed.To  balance  this  equation  we  require  1 moles  of  C2H4,  3moles  of  O2  to  form 2  moles CO2  and 2 moles  H2o
therefore the  equation is  represented  as
 C2H4+3O2 ->2CO2+2H2O

C2H4 + O2 → CO2 + H2O

We want the same number of each element on each side. 
Since there is only one compound on each side, we can start with carbon: When we double C02, we end up with 2 carbon atoms on each side. 
When we double H20, we end up with 4 hydrogen atoms on each side.
The right side now contains 4 + 2 = 6 oxygen atoms, so we put a 3 in front of 02.
The result is:
C2H4 + 3O2 → 2CO2 + 2H2O