475/500 = .95. So, you have the data from 95% of your sample, which is normally distributed.
In a Normal curve, 95% of your data lies between -2 and +2 standard deviations from the mean.
The mean of your newborns' weight is (2500+4000)/2 = 3250. This is the center of your normal curve.
The difference between the mean and -2 standard deviations is 3250-2500 = 750. Divide this by 2 to get your standard deviation of 375.