Answer
$280^{\circ} C$.
Work Step by Step
The maximum efficiency (Carnot efficiency) is given by equation 15–5. Make sure to measure temperatures in kelvins. Find the intake temperature from the original Carnot efficiency.
$$e_{ideal}=1-\frac{T_L}{T_H}$$
$$T_H=1-\frac{T_L}{1-e}=\frac{340+273K}{1-0.36}=958\;K$$
Now calculate the new low exhaust temperature for the desired Carnot efficiency of 0.42, using the intake temperature of 958 K.
$$e_{ideal}=1-\frac{T_L}{T_H}$$
$$T_L=T_H(1-e)=(958K)(1-0.42)=556\;K\approx 280^{\circ} C $$