Explanation:
"warmer" means the temperature is higher. so, we need to add the degrees.
on Thursday we have therefore 0 + 7 = 7°F.
this is still not "warm" for a human being, but it is "warmer" than (in the sense of "not as cold as") the original 0°F.
you see, so many basic things in math are just common sense. not rocket science ...