A function and its mirror image

An inverse undoes a function, and its graph is the mirror image in $y = x$. You will write an inverse, test the round trip, and check that mirrored points land on the inverse.

The idea. For $f(x) = 3x - 2$, solving $y = 3x - 2$ gives $x = \dfrac{y + 2}{3}$.

Your tasks.

1. In f_inv, return the inverse of $f(x) = 3x - 2$.
2. Set round_trip to True if f_inv(f(x)) == x for every x in xs.
3. Fill mirrored with the mirror images $(f(x), x)$ of the points of $f$, and check that each lies on f_inv.