Can someone help me plz

In the rectangle below ac=30 units what is de

Use circumcenter theorem to solve for the given values

Scores on the cognitive scale of the bayley-scales of infant and toddler development range from 1 to 19, with a standard deviation of 3. calculate the margin of error in estimating the true mean cognitive scale score, with 99% confidence, if there are 12 infants and toddlers in a sample.