This problem is fairly easy, if you are unable to solve this I recommend you to study the theory of carnot engines again.
If you got the theory and the formula right, you might be doing the mistake of not converting Celsius to Kelvin.
First case
η = 0.5
T_low = 273 + 7 = 280
Find T _highSecond case
η = 0.7
T_low = 273 + 7 = 280
Find T _high
Subtract T_high values of 2 cases and you will have your answer.