Consider the nitration of benzene using mixed conc. H _ 2 S O _ 4...

$Q = \dfrac{\text{Product}}{\text{Reactant}}$ as product increase $Q > K$

$Q$ has became equal to $K$ product has to decrease so equilibrium will shift backward direction.

If we add $KHS{O}_{4}^{-} , \text{conc}\text{.}HS{O}_{4}^{-}$ increases, equilibrium shifts backward.