There's an unfortunate assumption that your teacher made, but isn't shown explicitly. As such, I'd say that this problem is sufficiently flawed to consider it a fatal flaw. Yet I do see it happen in textbooks, all too often. That assumption is that the ground reference is the positive end of E3. It's not shown as such in the schematic. And that's fatal when asking a highly technical question and expecting highly technical answers. It means I can personally pick any node I want as ground and then tell the teacher that they are wrong in getting their answer. ;) Voltage is always relative.
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