Given the first number is 15.
Second number is 21.
Difference between the first and the second number is 6.
Now consider the second number and third numbers.
ie;21 and 57
Now the difference between the third and second number is 57-21=36,which is double the difference between the first and second number.
So the fourth number will be a number which if we subtract from 57 will get as 36*2
Therefor the fourth number is 36*6