.
This phrase
"During a 12 hour period, the tides in one area of the Bay of Fundy cause the water level to rise to 6m above average sea level
and to fall 6m below average sea level"
doesn't describe precisely what happens during this 12-hours period.
One can read/understand it as
"During a 12 hour period, the tide in one area of the Bay of Fundy changes water level from 6m above average sea level
to 6m below average sea level"
and then 12 hours is only half of the tide period, while the entire tide period is 24 hours.
So, this problem (or this difference in reading) is not about the Math.
It is about the exact meaning of the text,
which is, unfortunately, not clear enough to make a unique conclusion.
From the math point of view, your formula is correct UNDER THE ASSUMPTION that the period is 12 hours.
If the period is 24 hours, then simply replace 12 by 24 in your formula.
THE SECOND OPINION (and the third, fourth and so on) is desired.
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And one more IMPORTANT NOTE: in your formula for y (y = . . . ) you MUST explain what is "x".
I understand that it is time, but it must be written/pronunciation directly and explicitly: "x is time in hours, counting from the midnight".