An evildoer has stolen an unstable atomic nuclear object from a nuclear facility. While waiting at a stop light, the unthinkable happens (actually quite expected and predictable up to a probability), and the mass disintegrates into three particles. One of the pieces moves along the positive direction of the y-axis, and the other one moves along the positive direction of the x-axis. Find the magnitude and direction of the velocity of the third piece with respect to the negative direction of the x-axis.