For an incompressible fluid, the velocity components ( $u, v$, and $w$) are given as:
$\begin{array}{rcl}
u(x,y,z) & = & 2x + y + 2z \\
v(x,y,z) & = & ax + by + cz \\
w(x,y,z) & = & -6z
\end{array}$
The value of $b$ is $\_\_\_\_$. (Answer in integer)
