Why do you need the 21V in the first place? Just curious.
If you're going to be operating at approximately constant currents and very strict regulation on 21V isn't required, you can add a few diodes. But you must keep in mind the associated power dissipation.
If the load will be constant, you could also use a resistor to drop the voltage.
These are both linear regulation, voltage reduction actually (not regulation really), techniques. You'll have to deal with power dissipation of about 60W!
You should use a switching regulator for better efficiency. The downside would be the cost and complexity involved.
Hope this helps.
Tahmid.