Well, we want to use the equation of motion in classical physics, and that is the second law of motion. So we know that the sum of all the forces acting on that particle along the x axis is equal to the mass of that particle multiplied by its acceleration. Now, let's suppose that the velocity of that object, that particle, is constant. And what that means is if the velocity is constant, then the acceleration is essentially equal to zero. And so that implies that the thumb of the forces acting on that object along the x axis is equal to zero. Now, with that in mind, let's actually see what the left side of this equation is.