you need to understand that "evenly distributed" here means evenly spaced, which means that a,b,c d are in AP.

b-a = c-b
2b= 2^18+2^17

hence b= 3.2^16

common difference will be 2^18-3.2^16
= 2^16

hence d = 2^18+ 2^16 = 5.2^16

thanks for the reply but can you please explain in detail..

No problem runzun here it goes.

b-a= c-b
2b=c+a

putting values (c=4^9 = 2^18)

2b= 2^18+2^17
= 2^17(2+1) [taking 2^17 common]
2b= 3.2^17
b= 3.2^17/2
= 3.2^17.2^-1
= 3.2^16 [powers added]
common diff = c-b

2^18-3.2^16
2^16(2^2-3) [ taking 2^16 common]
2^16 (4-3)
2^16

now d will be c+ common difference
2^18+2^16
2^16(2^2+1)
2^16(4+1)
5.2^16

hope it helps....

thanks ...
it was great help