If f(f(x)) = x then you can say that if y = f(x) then x = f(y). So if the point (x,y) is on the graph of the function, then so is the point (y,x). The graph has x <-> y as a symmetry.

For the second, you're bieng asked what theinverseof the transformation x -> (-1/3x) does to the interval (a,b) and what the transformation y -> y-1 does to the interval (c,d).