A straight wire is placed in a magnetic field that varies with distance $x$ from origin as $\vec{B} = B_0 (2 - x/a)\hat{k}$. Ends of wire are at $(a, 0)$ and $(2a, 0)$ and it carries a current $I$. If force on wire is $\vec{F} = I B_0 (k a / 2)\hat{j}$, then value of $k$ is