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
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.