87 degrees minus 10 degrees would give you 77 (which is the lowest temperature). then 77 degrees plus 17 degrees is 94 degrees. the difference between the highest temp and lowest (94-77) would be 17.
Assuming it's a constant rate, there is a four hour difference from t=1 and t=5, so subtract the difference in depth (0.03 - 0.06) and divide that by four.
-0.03/4 = -0.0075 So your change in depth in feet per hour is -0.0075.