I do these types of problems a little different then some may. I turn this into a positive and see how far it is like that.
When thinking of it like that, how many numbers are between 45 and 71?
Just subtract 71-45= 26.
So know we know that there are 26 numbers in between 45 and 71.
If you go back to the original problem, you will see to get from -45 to -71, you have to -26.
So the temperature has to drop 26 degrees making your change in temperature -26 degrees.