So, In my precalc class we're learning whether or not a function is odd. And I keep on seeing this function notation -f(x) how do i evaluate it, is it just the opposite of the function? or is it just -1[f(x)]?

And what about if i'm trying to find out if a conic section is symmetrical to the origin, for instance an equation like x^2 + y^2 = 25, how do I do this -f(x) on an equation in which y isn't separated from the rest of the variables?