Oxidation of NO to NO2 ― one of the reactions in the production of smog ― appears to involve carbon monoxide. A possible mechanism is

CO + * OH --> CO2 + H *
H * + O2 --> HOO *
HOO * + NO --> * OH + NO2

(The formulas with stars(*) represent extremely reactive species with unpaired electrons and are called free radicals).

I need to write the net chemical equation for this reaction.

Please help me how to do it..

CO + O2 + NO ---------> CO2 + NO2?

please help

Yup, that's right. In fact, the H*, HOO* and O*H are acting as a catalyst here. Since free radicals are very reactive, the reaction is very fast. Without the free radicals, the reaction would be slower.