The answer is: [tex] g(t)=4sin(\frac{\pi}{6}x) [/tex] and the graph is shown in the image attached.
The explanation for this problem is shown below:
1. You have that the high tide was about [tex] 4ft [/tex] above average sea level, and the low tide was about [tex] 4ft [/tex] below average sea level. So, the amplitude of the function must be [tex] 4 [/tex]:
2. The formula for calculate the amplitude from a graph is:
[tex] A=\frac{maximum-minimum}{y2} [/tex]
3. As you can see in the graph attached, the maximum is [tex] 4 [/tex] and the minimum is [tex] -4 [/tex]. Then, the amplitude is:
[tex] A=\frac{4-(-4)}{2}=4 [/tex]
4. The amplitude of the sine function is the absolute value of [tex] a [/tex] in [tex] y=asin(bx+c)+d [/tex]. Therefore the amplitude of the function mentioned is: [tex]a=4[/tex] and the period is [tex] P=\frac{2\pi}{\frac{\pi}{6}} =12 [/tex], which corresponds with the period shown in the graph (the repeating of the pattern of the wave).