#### Answer

$H=3\sqrt {2}$ cm

#### Work Step by Step

The unknown length is the hypotenuse (H). The base (B) and perpendicular (P) are 3 cm each.
$H^{2}=B^{2}+P^{2}$
$H^{2}=3^{2}+3^{2}$
$H^{2}=9+9$
$H^{2}=18$
$H=\sqrt {18}$
$H=\sqrt {9\times2}$
$H=3\sqrt {2}$ cm