Answer:
Common Difference = 2.
Step-by-step explanation:
An AP can be written as  a1,  a1 + d,  a1 + 2d,  a1 + 3d,  a1 + 4d,  a1 + 5d, a1 + 6d , a1 + 7d.    
 where a1 = first term and d is the common difference.
Here first term = a1 = 8
3rd term = a1 + 2d = 8 + 2d
5th term = a1 + 4d = 8 + 4d
8th term = 8 + 7d
First 3 terms of a GP are  a , ar and ar^2
So from the given information:
a = 8 + 2d
ar = 8 + 4d
ar^2= 8 + 7d
Dividing the second equation by the first  we have
r = (8 + 4d)/(8 + 2d)
Dividing the third by the second:
r = (8 + 7d) / (8 + 4d)
Therefore, eliminating r we have:
(8 + 4d)/(8 + 2d)  =   (8 + 7d)/(8 + 4d)
(8 + 4d)^2 = (8 + 2d)(8 + 7d)
64 + 64d + 16d^2 =  64 + 72d^ + 14d^2
2d^2 - 8d  = 0
2d(d^2 - 4) = 0
2d = 0 or d^2 = 4, so
d =  0, 2.
The common difference can't be zero so it must be 2.