A cannon is fired horizontally from a platform (see figure). The platform rests on a flat, icy, frictionless surface. Just after the shell is fired and while it is moving through the barrel of the gun, the shell (mass 2.8 kg) has an acceleration of +2275 m/s2. At the same time, the cannon has an acceleration of -0.69 m/s2. What is the mass of the cannon