System one is a chair sitting next to the railroad track. The train is system 2. The train is moving to the left at 50 mph (relative to the chair) and a plane flies overhead left to right at 110 mph relative to the chair. What is the speed of the plane from the perspective of the train? Get that worked out using the mathematical notation above. Then got to a 2-D case where you are imagining a chair next to a lake and a boat moving out on the lake.