Hints you were given are very good, hard tot ell which part makes it impossible for you to solve the problem.
Would you be able to solve the problem starting from 0.01 M solutions of both substances?
First I calculated the dilution to determine the total amount of phosphate stock required:
0.01 x 100 = 0.2V
V = 5 ml of phosphate stock
Secondly, I used Henderson HasselBalch's equations:
pH = 6.6 Pka = 7.2
6.6 = 7.2 + log([base]/[acid])
-0.6 = log([base]/[acid])
0.25 =([base]/[acid])
Therefore there ratio of base to acid is 0.25:1
Therefore, of the 5 ml of stock solution, there is 1 ml of base (NaHPO4), and 4 ml of acid (NaH2PO4).
Is this correct??