In one day, there are two high tides and two low tides in equally spaced intervals. the high tide is observed to be 6 feet above the average sea level. after 6 hours pass, the low tide occurs at 6 feet below the average sea level. in this task, you will model this occurrence using a trigonometric function by using x as a measurement of time. assume the first high tide occurs at x = 0. what are the independent and dependent variables

 The "independent variable" is the "time" (in hours) that is plotted on the "x-axis".  Note the "time" (in hours) can be "manipulated" in the sense that is it "chosen" by the humans who are measuring the data (e.g. when to start, how many hours, and at what intervals.

The "dependent variable" is the measurement (in feet) above or below the sea level, and this is plotted on the "y-axis".  These values cannot be "manipulated" in the sense that one cannot "choose" what value or measurement the tide level would be at any particular time.

Hope these answers—and explanations—are of help to you.
Best wishes!