Suppose that a region $R$ has area $A$ and lies above the $x$-axis. When $R$ is rotated about the $x$-axis, it sweeps out a solid with volume $V$. When $R$ is rotated about the line $y=-k$ (where $k$ is a positive number), it sweeps out a solid with volume $W$. Express $W$ in terms of $V$, $k$, and $A$.

$W$ =