6.40 Given a quadratic function $y=f(x)$ whose graph has a vertex at $(-1,1)$ and intersects the $y$-axis at $(0,2)$.
(1) Find the expression for this quadratic function;
(2) When $x=8, y=$ ?
(3) For any given $y$ value, can we always find an $x$ value? Why? Explain using a graph.