def tf_solve(self, fn_x, *args):
raise NotImplementedError<EOL>
Solves an equation/optimization for $x$ involving an expression $f(x)$.

Args:
    fn_x: A callable returning an expression $f(x)$ given $x$.
    *args: Additional solver-specific arguments.

Returns:
    A solution $x$ to the problem as given by the solver.