Explanation:

The series follows a pattern of multiplying the number with a prime number and subtracting the same from the resultant after multiplication, prime numbers in descending order starting from 11.

*11-11 , *7-7 , *5-5 , *3-3 , *2-2

10 *11-11= 99 

99*7-7=686

686*5-5=3425

3425*3-3=10272

10272*2-2=20542