At a certain temperature, the equilibrium constant for the following chemical?

IDK WHAT IM DOING AT THIS POINT ANYMORE HELP???

At a certain temperature, the equilibrium constant for the following chemical equation is 3.20. At this temperature, calculate the number of moles of NO2(g) that must be added to 2.86 mol of SO2(g) in order to form 1.30 mol of SO3(g) at equilibrium.

SO2+NO2<-->SO3+NO

_

_______mol of NO2 (g)

i got 0.51 but its saying its wrong that :The number of gaseous products equals the number of gaseous reactants and so Kp = Kc = 3.20. This means that instead of having to use concentrations or pressures, you can leave the quantities in moles in the K expression