Remember mole concentration = number of moles /number of liters of solution.
35.0ml of 0.150M NaCl contains 0.035 times 0.150 = 0.0525 mol NaCl thus 0.00525 mol of Na+ ions
75.0m of 0.050M Na2SO4 contains 0.075 times 0.050 = 0.00375 mol Na2SO4 thus 0.0075 mol of Na+ ions
Total moles of Na+ = 0.00525 +0.00750 = 0.01275 mol
Total volume of solution = 35.0ml + 75.0ml = 120 ml = 0.120 l
Thus mole concentration of mixed solution = 0.01275 mol/0.120 l = 0.10625 M = 0.106M (in 3s.f)
If you need the concentration in grams per liter of solution, just multiply by molar mass of sodium.