Three remarks. First, there are few natural processes that move in straight lines and angles. Tides are more likely to change in a sinusoidal way. Search the web for tide graphs.
Second, it is not clear whether "the height of the tide," which you need to draw, means the absolute depth of the water or the depth minus the average sea level. Since the absolute depth at low tide is given, I would guess it is the former. So, the minimum would be 2m.
Third, again, it is not clear whether the phrasemeans that the 12-hour period ends at low tide or average level. Literally, it seems to mean the former, though I would not be surprised if the situation where the whole period takes 12 hours is described this way.During a 12hour period, the tiles in one area of the bay of Fundy cause the water level to rise to 6 m above average sea level and to fall 6 m below average sea level