A particle of mass m and charge q moves with a constant velocity v...

A particle of mass $m$ and charge $q$ moves with a constant velocity $v$ along the positive $x$-direction. It enters a region containing a uniform magnetic field $B$ directed along the negative $z$ direction, extending from $x=a$ to $x=b$. The minimum value of $v$ required so that the particle can just enter the region $x>b$ is