A buffer is prepared consisting of 0.75 mol of sodium dihydrogenphosphate, NaH2PO4, and 0.60 mol of sodium hydrogenphosphate, Na2HPO4, dissolved in 600.0 mL of water. The Ka for the dihydrogenphosphate

Answer 1

You didn't finish but I assume you want to know the pH of the solution. Use the Henderson-Hasslebalch equation.
pH = pKa + log (base)/(acid). The base is Na2HPO4 and the acid is NaH2PO4. Substitute and solve for pH. Post your work if you get stuck.