Answer:
a = 4
Explanation:
You are given an arithmetic sequence with second term 5.5 and fifth term 10. You want to know the first term.
Common difference
The 2nd term will have 3 times the common difference added to it to make the 5th term. Then that difference can be found from ...
5.5 +3d = 10 . . . . using 'd' for the common difference
3d = 4.5
d = 1.5
First term
The second term (a2) is obtained by adding the common difference (d) to the first term (a1):
a2 = a1 +d
5.5 = a1 + 1.5
4.0 = a1
The value of 'a' is 4.
__
Additional comment
The 'd' we're using here to represent the common difference is not to be confused with the 'd' in the problem statement that represents the fourth term of the list.
The fact that adjacent list members have the same difference is what tells you this is an arithmetic sequence. In terms of the problem statement, you have ...
{a, b, c, d, e} = {4, 5.5, 7, 8.5, 10}
Here, we have used the more common notation for elements of an arithmetic sequence.