To find height $ h $ use Pythagorean Theorem:

$$ h^2 + \frac{ a^2 }{ 4 } = s^2 $$

After substituting $a = 18\, \text{cm}$ and $s = 12\, \text{cm}$ we have:

$$ h ^ {\,2} + \frac{ \left( 18\, \text{cm} \right)^{2} }{ 4 } = \left( 12\, \text{cm} \right)^{2} $$ $$ h ^ {\,2} + \frac{ 324\, \text{cm}^2 }{ 4 } = \left( 12\, \text{cm} \right)^{2} $$ $$ h ^ {\,2} + 81\, \text{cm}^2 = \left( 12\, \text{cm} \right)^{2} $$ $$ h ^ {\,2} = 144\, \text{cm}^2 - 81\, \text{cm}^2 $$ $$ h ^ {\,2} = 63\, \text{cm}^2 $$ $$ h = \sqrt{ 63\, \text{cm}^2 } $$$$ h = 3 \sqrt{ 7 }\, \text{cm} $$

Pyramid Calculator