What the above statement is saying is: If the number is 4, find the difference between the square of the number (the number is 4) and the number itself (the number is still 4).
The square of the number is: The number^2 = 4^2 = 4 × 4 = 16
The difference between the square of the number and the number is: The number^2 - The number = 16 - 4 = 12