A brass wire of length 2 m and radius 1 mm at 27^ C is held taut...

Thermal stress in the wire is caused by contraction.
At -43°C, the temperature drop from 27°C is $\Delta T_1 = 70°C$
It creates tension $T = YA\alpha\Delta T_1$ where $Y$ is Young's modulus and $\alpha$ is linear expansion coefficient.
For tension to increase to $1.4T$, the new temperature drop must be: $\Delta T_2 = 1.4 \times 70 = 98°C$.
Required temperature: $T_3 = 27 - 98 = -71°C$