I would think that Un + m would equal 2m since Un = m. Could you explain why that is not the case?
You typed part (b) incorrectly . . .
The term is given by: . . where: .For a given arithmetic sequence: .
. . . . .
We have: .
Equate  and : .
. . . . . .
If , we have: .
. . and the arithmetic sequence cannot be determined.
If , then: .
. . . .
Since , we have: .