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Find the 7^{th} term of the sequence using the explicit rule provided.

Plug the first term, seventh term, and common difference into the partial sum formula for arithmetic series.

Simplify to obtain your solution.

Example 2

Evaluate:

This is the same thing as being asked to find the 14^{th} partial sum of {a_{n}} = -4n + 20. All we need to do to evaluate this partial sum is to find the number of terms as well as the first and last terms.