Find the concentration of the polyprotic weak acid given the following:

Question

- 1 equiv needs 6 ml NaOH, 2 equiv needs 18 ml NaOH, 3 equiv needs 28 ml NaOH
- pKa1 = 2, pKa2 = 3.9, pKa3, 10
- Molarity of NaOH = 0.175
- Total volume of NaOH = 28 ml
- Volume of weak acid = 30 ml

Please help! Thanks!