In ISA, the tropopause is at 36.090 feet and the temperature is -56.5oC. Above the tropopause, in the lower stratosphere the temperature would remain constant at -56.5oC.

In the question as you have -57oC at FL390, if the tropopause would be at FL 360 in ISA conditions, then at FL 360 the temperature would also be -57oC. Only below FL360 would the temperature being to increase, therefore: -57 + (36 x 2) = 15oC at SL => ISA DEV is 0.