Answer:
Common difference(d) 
(21) -10 -548
(22) -7 -323
(23) 10 547
(24) -100 -5118
Step-by-step explanation:
Let the common difference be denoted by 'd'.
Also the nth difference of an arithmetic sequence is given by: 
(21)
We are given a recursive formula as:

The first term is given by:

The common difference for an arithmetic sequence is given by:

Hence, here we have the common difference as:

The nth term of an arithmetic sequence is given by:

Here
and
.
Hence, 
Hence, 
(22)


The common difference for an arithmetic sequence is given by:

Hence, here we have the common difference as:

Here
and
.
Hence, 
Hence, 
(23)


The common difference for an arithmetic sequence is given by:

Hence, here we have the common difference as:

Here
and
.
Hence, 
Hence, 
(24)


The common difference for an arithmetic sequence is given by:

Hence, here we have the common difference as:

Here
and
.
Hence, 
Hence, 