We are given "Nicolas has three fewer than twice the number of songs as Sabrina does".
Here x variable is taken for number of songs downloaded by Sabrina.
Nicola has three fewer than twice of Sabrina.
Twice the number of songs downloaded by Sabrina = 2 times of x = 2x.
Three fewer than twice the number x could be written as 2x-3 because, 3 fewer represents 3 less than of 2x.
Therefore, 2x-3 represents " three fewer than twice of x".