The bar graph shows the percentage of U.S. adults who have been tested for HIV, by age.Per...

The bar graph shows the percentage of U.S. adults who have been tested for HIV, by age.

Percentage of Adults in the United States Tested for HIV. by Age

Source: National Center for Health Statistics

The data in the graph can be modeled by the formula

P = −0.05x2 + 3.6x − 15,

where P represents the percentage of U.S. adults tested for HIV at age x. According to the formula, what percentage of U.S. adults who are 21 years old have been tested for HIV? Does the model underestimate or overestimate the percent displayed by the bar graph? By how much?