Any change in speed or direction requires acceleration and application of a force causes acceleration. So you can break this problem into two steps: 1) is there a force acting here? 2) if so, is the resultant acceleration causing a change in speed or direction (the two components of the velocity vector)? So to tackle (1), you have to know which force is relevant here. Since it's a charged particle moving through a magnetic field, it must be the magnetic force, given by F = q*(v x B), where v and B are both vectors. So you have a charge, q, that has some velocity, v, and it's moving through a magnetic field, B. Therefore, there must be a magnetic force on the particle. Now, let's tackle (2). What do you know about the direction of magnetic force, or the direction of a vector that is the resultant of a cross product? The direction of the resultant vector must be perpendicular to the two vectors being crossed and is given by the so-called right-hand rule. You don't really have to apply the right-hand rule in this case because the first part of the last sentence should be enough to answer the question, namely "the resultant vector must be perpendicular to the two vectors being crossed." If a force is applied always perpendicular to the direction of motion (the direction of vector v), then that force will never change the speed of the particle. Think of it this way - if the force is applied perpendicular to the direction of motion, then work must equal zero (W = F*d*cos(theta), theta = 90 in this case). But there is a net force that that force is going to act to change the direction of the particle - this very force is what causes the particle to be traveling at a right angle to the field, or orthogonal to it.