For example if $$ f(x)= \Theta (g(x)) $$
from the definition of the theta notation, there exist c1 and c2 constants such that
$$c_1 g(x) \le f(x) \le c_2 g(x)$$
then if only we took the constants $1/c_1$ and $1/c_2$ we could say from the definition that
$$ g(x)= \Theta (f(x)) $$
Right?