As a proton passes the origin, its velocity is $3.0\times {10}^5\mathrm{m}/\mathrm{s}$ in the positive x direction. What is the direction of the magnetic field at the point ($12.0\mathrm{cm}$, $5.0\mathrm{cm}$, $0.0\mathrm{cm}$)?

Use the cross product: The magnetic field direction is given by the cross product of the velocity vector and the position vector. Since the velocity is in the x direction, the cross product will result in a vector in the positive z direction, confirming the direction of the magnetic field.