In assemblying the nucleosome, this reaction occurs in two main steps. the H3 and H4 are recruited first to the DNA in pairs forming the H3/H4 tetramer; meaning two of H3 and two of H4. This gives rise to the nucleosome precursor. Then after this, the dimers of both H2A/H2B are recruited to this precursor, to give rise to the octamer structure around which the DNA is wrapped.
It looks like the reactants are not allowed to have coefficients (not enough boxes), so you need to pick reactants and products that satisfy this condition. The only ones that appear to fit are:
