A ratio here would be the measurement of the same side in the different polygons. So,
Side A : Side B
2.8:7
And
2:5
To check if they are similar figures simply divide any of the sides by another side. Do that do the other polygons too and see if you get the same answer.
Side B / Side A
So, 7/2.8 = 2.5
And 5/2 = 2.5
Since they are the same answer, this means that they are similar because they have the same ratio.
Hope this helped!
~ Luna