make a projection of the incidence rate of diabetes mellitus during the period 1995−1999, and provide a 95% confidence interval about this estimate.

So this is the question and the prediction i made for the incidence is using the formula (Incidence = 184.9+33.56*time-period score) and my answer is 386.26 with a time period of 6. How do i calculate a confidence interval based on this information?
Thank you