How do you write the phrase as a variable expression: the product of -6 and the sum of a number and -1?
You can consider "the sum of a number and -1" as a single entity.
So the question is asking the product of -6 and sum of a number and -1
Since, we don't know what the number is, we assume it as a variable (say x).
Hence the answer.
Hope it Helps :)