Not exactly sure which equation you meant:
3(100-b)=6b
300-3b=6b
300=9b
b= about 33
100-3b=6b
100=9b
b=about 11
The answer is 55% because 1,100 is 55% of 2,000
Do you mean subtract two each day? If so...
4x3=12 and 12 divided by 2 is 6 so it would take six days to reach zero
or
you could manually subtract two from 6 each days as in 12-2=10 (1 day), 10-2=8 (2 days), and so on...

Now, If the exponent was negative like you asked....