According to the given reaction, one molecule of H2 reacts with one molecule of C2H4 to produce one molecule of C2H6.
In the reactants, there are 7 molecules of C2H4 and 3 molecules of H2, it means that 3 molecules of C2H4 react with the 3 molecules of H2 to form 3 molecules of C2H6, and 4 molecules of C2H4 will remain.
It means that you have to add 3 molecules of C2H6 and 4 molecules of C2H4.
