On a particular lake, the difference between high and low tide is 6 hours. At low tide, 6 a.m., a student measures a 1.4 m distance from the dock she is standing on, down to the water. Two hours later, she measures the distance from the dock down to the water to be 1.29 m. At 9 a.m., the distance is 1.175 m down to the water.

My teacher is asking me to determine a function for the distance from the dock down to the water with respect to the # of hours since midnight.

From this I've formed the equation 0.8125cos(pi/6x)+c. How do I find the vertical shift with the information I have?