A particle of rest mass Mo is at rest in the laboratory when it decays into three identical particles, each of rest mass mo. Two of the particles (1 and 2) have velocities and directions as shown. [1 is 3c/5 at -90degrees, 2 4c/5 at 180degrees]
(a) Calculate the direction and the speed of...
That's good; now you want x to be a minimum to get that particular point so take the derivative - remember at a minimum the derivitave = 0. A little trigonometry and the angles dissappear.