Red Orange Yellow Green Blue Indigo Violet
<---Longest wave length shortest wave length--->
As you went down the colors of the rainbow the wave length got shorter or faster.
Answer:
H₂ + Cl₂ ---> 2 HCl
Explanation:
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:
H₂ + Cl₂ ---> 2 HCl