Why is this incorrect (special relativity time dilation)

Usually, problems of this type assume that observers correct for the lightspeed delay. That means that the flashbulb goes off at 100ns and Anna actually sees the flash a bit later. Anna was kind enough to subtract off the "bit later", so you know when the flash went off, not when Anna saw it.

Your answer is wrong because you are assuming that the flash went off at t=0 and Anna saw it at t=100ns. That's not what the question says.

Bob thinks the flashbulb went off at a different time because he is in a different reference frame. What equations link positions and times in different frames?

 

It's not so confusing. Think about it like this: you see a lightning flash and hear the thunder five seconds later. When would you say the sound was emitted?

You need to read the question carefully. It does say that the clock reads 100ns when the pulse was emitted. It also doesn't say where Anna's eyes are. Is she the same height as Bob? The question would need to specify all that and more if it were talking about the times Anna and Bob saw the flash.

In other words, this question only talks about the pulse being emitted, not being received.

Does that help?

 

Are you sure that's the right answer? That would make her arm 18 meters long.