the missing blank is filled by 11 O2
that is
= 2C4H6 + 11O2 = 8 CO2 + 6H2O
C4H6 reacted with oxygen (O2 )through the process of combustion to form carbon iv oxide (CO2) and water (H2O) . 11 infront of O2 is to make sure the molecules of O2 is balanced in both reactant side and the product side