I don't know the identity of HO2. The OH needs a minus sign (OH^-). Cr goes from +3 on the left to +6 on the right. I don't see a reduction but it may be the HO2 (could that be H2O2?). Here is a good site that shows you everything you may need to know about redox.
http://www.chemteam.info/Redox/Redox.html
