You want to find the half-equivalence point. Whatever volume of strong base was required for the equivalence point, take half that volume. Find the pH associated with that half volume amount. The pH at the half-equivalence point is equal to the pKa. We know from this:
pH = pKa + log (salt of weak acid / weak acid)
At the half-equivalence point the salt of weak acid and the weak acid are equal amounts. That makes the log term become zero, leading to this:
pH = pKa
antilog the pKa for the Ka.