Two distinct solid fuel propellants, type a and type b, are being considered for a space program activity. burning rates of the propellant are crucial. random samples of 12 specimens of propellant a are taken with sample means of 84 cm/sec with a standard deviation of 4. and, random samples of 18 specimens of propellant b are taken with sample means of 77 cm/sec with a standard deviation of 6. find a 99% confidence interval for the difference between the average burning rates for the two propellants. assume the populations to be approximately normally distributed with equal variances.