a 6.77 $\mu c$ particle moves through a region of space where an electric field of magnitude 1280 n/c points in the positive $x$ direction, and a magnetic field of magnitude 1.35 t points in the positive $z$ direction. if the net force acting on the particle is 6.12e-3 n in the positive $x$ direction, calculate the magnitude of the particle's velocity. assume the particle's velocity is in the $x$-$y$ plane.