The bridge formed with given resistances is a balanced Wheatstone's bridge The situation can be depicted as shown in the figure.
As resistances S and 6Ω are in paralle, their effective resistance is 6+S6SΩ. For balancing condition, QP=(6+S6S)R or 22=6 S2(6+S) or 3 S=6+S⇒S=3Ω