Assume KOH and HCl react first - that will just remove KOH and some HCl, simplifying the problem.
Then, list all pKa values involved (4 of them matter). At pH=7.21, which bases will be fully protonated, which will be not protonated at all, and which will be protonated partially? To what extent?
How much acid - in total - do you need to get to this situation? (Once you calculate amounts of bases protonated, this will be a simple stoichiometry). How much acid (HCl) do you have? ANd how much acid (HNO3) do you need to add?
After neutralizing KOH, HCl remains 0.01 mole.
I think that PO43- (which the max Kb ) fully to be protonated.
So, that will be 0.0102 mole HPO42-and use all HCl to neutralize.
If I right, OH- remaining =0.0002 ----*
For HPO42- be protonated partially to form H2PO4-
Hence ,for pH =7.21 = pka +log base/acid =7.20+log([H2PO4-]/[HPO42- ]) ---------1)
CN- be protonated partially to form HCN?
Hence, pH =7.21= 9.3+log([HCN]/[CN]) --------2)
In this step, we will find the amount of H+ in ICE of these two ions ?
Then from *,1) ,2) we will get all amount H+ ion.
And we will get the amount of HNO3 ?