REPLACING all the independent voltage sources with short circuit and all the independent current sources with an open circuit.Calculation:Thevenin voltage:By applying the KVL,-50 + 10I + 15I + 100 = 0⇒ I = -2 AVTH = VAB = 50 – 10I = 70 VThevenin resistance:RAB = RTH = (10||15) = 6Ω