In a study dealing with the difference between the average pressure that two types of glass can withstand, samples of 12 units of glass of the first type and 6 units of glass of the second type were randomly selected. The mean of the first sample is 15 units, the variance is 6; The mean of the second sample was 16.4 units and its variance was 38. Under the assumption that the population variances are unknown and equal, which of the following is the 90% confidence interval for the mean pressure difference that the two types of glass can withstand