there are 2 pentagons

the small pentagon has a side that is 3cm while the side on the other larger pentagon is 8cm.

the small pentagon has a side x cm while the same side on the larger pentagon is 5cm.

would i set up a proportion 3/x = 8/5

2 answers

I think it is right
You are correct if the two pentagons are similar.
You did not mention if they are.