A farm has a large corn field with man-made ponds along the east and west borders. An environmental science student wonders whether the levels of a particular pesticide are the same in the two ponds. The student collects water samples from random locations and depths in each pond. The 35 samples from the pond to the east of the field have a mean level of 6.2 parts per billion (ppb) with a standard deviation of 2.7 ppb. The 35 samples from the pond to the west of the field have a mean level of 5.7 ppb and a standard deviation of 2.9 ppb. Determine the degrees of freedom for a hypothesis test for the difference between the two mean pesticide levels, assuming that the population variances are equal