This is a neutralization reaction between phosphoric acid and sodium hydroxide.
First write a balanced chemical equation:
3 NaOH + H3PO4 --> Na3PO4 + 3 HOH
note that it takes 3 moles of base to completely neutralize the acid.
Now you are starting with 15 mL of 0.2 M acid solution.
mL * M = millimoles
So you have 3 millimoles of acid in solution.
Based on the chemical equation, you need 9 millimoles of base.
9 = 0.1 * mL
9/.1 = 90 mL of NaOH needed