Correct. The length of the pendant is a hypotenuse of a right triangle. Call the length x. The length of both legs is 3 cm. Use the Pythagorean Theorem:

$\displaystyle{{3}}^{{2}}+{{3}}^{{2}}={{x}}^{{2}}\usepackage{cmbright}\Huge\$

$\displaystyle{18}={{x}}^{{2}}\usepackage{cmbright}\Huge\$

$\displaystyle{4.2}\approx{x}\usepackage{cmbright}\Huge\$