Hi there! I'm glad I was able to help you!
To rewrite these sentences and combine them using a semicolon, it would look like this:
Temperatures vary wildly on Mercury; they range from 810 degrees Fahrenheit on the sunlit side to -290 degrees Fahrenheit on the dark side of the planet.
When you use a semicolon, you're basically sandwiching two normal sentences into one compound sentence!
I hope this helped you! Feel free to leave a comment below, in case I can help you with anything else!