In a test to compare yield strengths of two grades of carbon steel bolts, strengths of eight grade 2 bolts and six grade 5 bolts were measured. The results, in MPa, were as follows: Grade 2 408 401 406 396 407 394 399 414 Grade 5 637 659 655 644 663 635 Find a 98% confidence interval for the difference between the mean yield strengths for grade 5 and grade 2 bolts. (Round down the degrees of freedom to the nearest integer and round the final answers to two decimal places.) The 98% confidence interval is ( -267.84 , -233.58 ).