Respuesta :

13 because you do 18-8 and you get 10 and divide that by 2 and you get 5 then add 5 to 8 and you get 13 and subtract 5 from 18 and you get 13 as well. Hope that helps
Since it's geometric, the ratio between the second and first term is equal to the ratio between the third and secons term.

So, x/18 = 8/x
x^2 = 144
So, x can be either 12, or -12