Trying to add GrossWeight each time it shows up in the list

Trying to learn how to use a ForEach loop while doing my homework. I feel like this should be easier than it is, but basically I am trying to add the grossweight by each other every time one is found in my list. Once I do that I need to add the weight of the engine and return the whole equation sum out of the method. If more code is needed please go ahead and ask, I'll provide.