For an acid $HA,$ the concentrations of $HA$ and $A^{-}$are $0\mathrm{.}076$ and $0\mathrm{.}038,$ respectively, at $pH6\mathrm{.}3.$ What is the equilibrium constant $K_a$ for this acid HA$?($Hint: Use HendersonHasselbalch equation$)$