Given the arithmetic sequence:

To determine the common difference between the terms of the sequence you have to calculate the difference between one term of the sequence and the previous one.
For example, you can determine the difference between the second and first terms of the sequence:

Where
d is the common difference
a is the first term of the sequence
a₂ is the second term of the sequence
For the given sequence the first term is a=8 and the second term is a₂=5, you can calculate the common difference as follows:

The common difference for this arithmetic sequence is d= -3