[Borek]

Geez, stop whining, start trying.

Assume volume of the acid and volume of the base are V_NaOH and V_H3PO4. Use these volumes to write formulas for:

Number of moles of the acid present initially.

Number of moles of the acid that were neutralized.

Number of moles of the acid that were left (hint: use formulas written above)

Final concentration of the acid.