Answer:
The temperature of the city a was 5 °C and b was 4 °C.
Explanation:
First we have to write the equations based on the description, this is:

Now that we have the equations, we can replace the second in the first:

Now we can find the value of b:





Now with the value of b and the second equation we can get the value of a:




The temperature of the city a was 5 °C and b was 4 °C.