use henderson equation

5) You are in charge of making a phosphate buffer at pH 7.5. You are out of solid versions of phosphate, but you find stock solutions of 1.0MNaH2PO4 and 1.0MNa2HPO4 in the lab and decide to make the buffer with these solutions. How much of each of the stock solutions would you mix together to make 200 mL of 1.0M phosphate buffer at pH7.5 ? (The pKa values of phosphoric acid are 2.1, 7.2, and 12.3.)