Sum calculation

[frosty summit]

I'm stuck, I don't see what I can do except extract the 5 from the sum, I need to calculate this but I don't know how.

[lilac goblet]

1. Wait patiently for a helper to come along.
2. Once someone helps you, say thank you and close the thread with:

+close

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4. Do not ping the mods, unless someone is breaking the rules.
5. If you're happy with the help you got here, and the server overall, you can contribute financially as well:

[steady coral]

Hint:
3^(k-2) = (3^k)/(3^2)
7^(k+2) = (7^k)(7^2)

[frosty summit]

righttttt thank you sm!

[frosty summit]

+close