sea levels follow a periodic pattern of rise and fall called the tides. the graph below shows the sea level…

Explanation:

Step1: Find the mid - line formula.

The mid - line of a periodic function $y = A\sin(B(x - C))+D$ (or other periodic functions like cosine) is given by $y = D$. For a graph of a periodic function with maximum value $M$ and minimum value $m$, the mid - line is $y=\frac{M + m}{2}$.

Step2: Identify maximum and minimum values.

From the graph, the maximum sea - level is $M = 48$ feet and the minimum sea - level is $m = 0$ feet.

Step3: Calculate the mid - line.

Using the formula $y=\frac{M + m}{2}$, we substitute $M = 48$ and $m = 0$: $y=\frac{48+0}{2}=24$.

Step4: Interpret the mid - line.

The mid - line $y = 24$ represents the average sea - level at the certain beach over one day.

Answer:

The mid - line is $y = 24$ feet, and it represents the average sea - level at the beach over one day.