The average is also known as the mean of the set of values given. If they want the average of the data set the formula would be to add all the numbers given and divide it by the amount of numbers there are (nth value).
Here we have 7 numbers which is our n. That will be the denominator. Let's add the given data set:
The numerator will be -7. Now put the numerator and denominator together to divide.
Therefore the average is -1 degrees Fahrenheit.
Hope that helps!