Asportswriter wished to see if a football filled with helium travels farther, on average, than a football filled with air. to test this, the writer used 18 adult male volunteers. these volunteers were randomly divided into two groups of nine subjects each. group 1 kicked a football filled with helium to the recommended pressure. group 2 kicked a football filled with air to the recommended pressure. the mean yardage for group 1 was = 30 yards with a standard deviation of s1 = 8 yards. the mean yardage for group 2 was = 26 yards with a standard deviation of s2 = 6 yards. assume that the two groups of kicks are independent. let μ1 and μ2 represent the mean yardage we would observe for the entire population represented by the volunteers if all members of this population kicked, respectively, a helium-filled football and an air-filled football. let σ1 and σ2 be the corresponding population standard deviations. assuming two-sample t procedures are safe to use, what is a 99% confidence interval for μ1 – μ2? (use the conservative value for the degrees of freedom, which is the formula.)