The length of the sides of a six sided polygon are an arithmetic sequence, the length of the hortest side is 3 meters. if the length of the next longer side is 5 meters what is the length of the longest side

How do you get from 3 meters to 5 meters?  Add 2 meters.  How do you get from 5 meters to the next measurement?  Add 2 meters again.

Start at 3 meters, add 2 to get 5 meters, and so on.  Continue this pattern until you have all 6 side lengths.  The last one represents the length of the longest side.

Here we're working on an "arithmetic sequence."