Help guys geometry isn’t my strongest subject rn

How do i simply this expression (quadratic formula basis) on a ti-84 or normal calculator?

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.

Use long division to indentify (12xto the second power+15x-8)divided by 3x