Explanation:
if you are not sure what a word means, use the general internet.
that is much faster than putting such a question in here and then waiting for an answer.
e.g.
a pentagon has 5 corners and 5 sides connecting these corners.
as penta is Greek for 5.
for a decagon, well, deca means 10.
so, a decagon has ... ... 10 corners and 10 sides.
the perimeter is the whole way around the object, so, it is the sum of all 10 sides.
therefore one side is
85 in / 10 = 8.5 in