Answer
$V=231.97\ mL$
Work Step by Step
By the Henderson-Hasselbach Equation:
$pH=pKa+\log([HPO_4^{2-}]/[H_2PO_4^-])$
$[HPO_4^{2-}]=0.5\ M\times 750.0\ mL/(V+750.0)\ mL-[H_2PO_4]$
$[H_2PO_4^-]=[HCl]=1.00\ M\times V/(V+750.0)$
$7.00=7.21+\log((375-V)/V)$
$V=231.97\ mL$
