Correct. The dime has a diameter of about 18 millimeters. Its radius is half of this. So substitute $\displaystyle{r}={9}\usepackage{cmbright}\Huge\$ in the formula $\displaystyle{A}=\pi{{r}}^{{2}}\usepackage{cmbright}\Huge\$ to find the area of one side.

$\displaystyle{A}=\pi{{\left({9}\right)}}^{{2}}\usepackage{cmbright}\Huge\$

$\displaystyle{A}=\pi\cdot{81}\usepackage{cmbright}\Huge\$

$\displaystyle{A}\approx{3.14}\cdot{81}={254.34}\approx{254}\usepackage{cmbright}\Huge\$ square millimeters

The area of one side of a U.S. dime is approximately 254 square millimeters.