Since the two legs have the same length, the two acute angles must be equal, so they are each $\displaystyle{{45}}^{\circ}\usepackage{cmbright}\Huge\$ .

In a $\displaystyle{{45}}^{\circ}-{{45}}^{\circ}-{{90}}^{\circ}\usepackage{cmbright}\Huge\$ triangle, the length of the hypotenuse is $\displaystyle\sqrt{{2}}\usepackage{cmbright}\Huge\$ times the length of the leg. So:

$\displaystyle{h}=\sqrt{{2}}\cdot{3}\sqrt{{2}}={3}\cdot\sqrt{{2}}\cdot\sqrt{{2}}={3}\cdot{2}={6}\usepackage{cmbright}\Huge\$