Since, area of square poster = 31 square inches.
We have to determine the length of one side of the poster to the nearest tenth of an inch.
Area of square with side 's' = [tex]s \times s[/tex]
So, [tex]31 = s \times s[/tex]
[tex]31 = s^2[/tex]
[tex]s = \sqrt 31[/tex]
s = 5.56
Now, rounding to the nearest tenth, we get the side of the square poster as 5.6 inches.
Therefore, the length of one side of the poster is 5.6 inches.