First, use "split" to get a list of the words in the sentence.
Make a for loop to go through each word in the list. You also need set a variable to count the total length of all the words by adding it to itself and the length of the word (using len()). You also need another variable to count the numbers of words.
`totalLength=totalLength + len(word)`
Then, the average length will be the total length /numbers of words