Answer:
First, let's define an arithmetic sequence:
In an arithmetic sequence, the difference between any two consecutive terms is always the same.
Then we can write it in a general way as:
aₙ = a₁ + (n - 1)*d
where:
aₙ is the n-th term of the sequence.
d is the constant difference between two consecutive terms.
a₁ is the initial term of our sequence.
Now in this case we know that the first terms of our sequence are:
84, 77, ...
Then we know the initial term of our sequence:
a₁ = 84.
And the value of d can be calculated as:
d = a₂ - a₁ = 77 - 84 = -7
Then the general way of writing this sequence is:
aₙ = 84 + (n - 1)*(-7)
And the recursion relation is:
aₙ = aₙ₋₁ - 7
So for the n-th term, we must subtract 7 of the previous term.