This is the complete list of proper factors of a number
what is the number

The factors are all the possible things you would multiply to get the number. So finding the 2 middle numbers, in this case is 21 and 28 and multiplying them, you get 588. If you're not sure if that's right, take the number on the left or right of the 2 middle numbers, 14 and 42. The product of these 2 numbers are also 588. So your number is 588.