What is the 95% t confidence interval for the mean BMI of all young women?

We developed a 95% z confidence interval for the mean body mass index (BMI) of women aged 20 to 29 years, based on a national random sample of 654 such women. We assumed there that the population standard deviation was known to be σ = 7.5. In fact, the sample data had mean BMI x = 26.8 and standard deviation s = 7.42.