A 2.1 kg toy tank is designed to shoot a 0.20 kg ball directly horizontally. The tank has the ball inside of it before it is launched. Find the speed of the tank if the ball is launched at a velocity of 7.9 m/s [N] relative to the ground. Before launch, the tank is travelling at 1.8 [N] m/s relative to the ground. Enter your answer WITHOUT a direction, but use a negative sign to denote a velocity in the