a) The recursive formula you've provided is solving for the term \( a_n \), where \( n \) is the position of the term in the sequence.
b) To determine what should be in place of \( n-1 \) in the term \( a_{n-1} \), we have to consider the recursive relationship given:
- The formula specifies that \( a_4 = a_{n-1} + 2 \).
For this relation to hold true, \( n \) must equal 4 since \( a_4 \) is being defined in terms of the previous term in the sequence. Therefore, \( n - 1 = 4 - 1 = 3 \).
So, the answer to (b) is 3.
Thus:
- a) The formula is solving for \( a_n \).
- b) The correct value to replace \( n-1 \) is 3 (making it \( a_3 \)).