since HSO4^- ⇋ H^+ + SO4^2- is in an equilibrium
we know that [H+] is 0.19055moldm^-3 so there has been an increase from the initial moles (conc.?) of 0 to 0.19055 and the same happens to SO4^2- since they are in a 1:1 ratio
as the conc of the products increase the conc of the reactant HSO4^- decreases by the same amount
HSO4^- ⇋ H^+ + SO4^2-
intial conc : 0.050375 , 0 , 0
change in conc : -0.019055 , +0.019055 , +0.019055
final conc : 0.03132 , 0.019055 , 0.019055
you then use that in the Ka equation Ka = [H+] [SO4^2-] / [HSO4^2-] giving us Ka = 0.0116 units mol dm^-3