Assuming the index start from [tex] 1 [/tex], i.e. the first element is [tex] a_1 [/tex], the answer is the last one: [tex] a_n = -5n+12 [/tex]
In fact, the elements of the sequence differ by [tex] 5 [/tex] units from each other, and are descending, which justify the [tex] -5n [/tex] part.
Nevertheless, the sequence starts from [tex] 7 [/tex], so when [tex] n=1 [/tex] we must have
[tex] -5+k=7 \iff k=12 [/tex]
hence the formula
[tex] a_n = -5n+12 [/tex]
