To find side $ l $ use Pythagorean Theorem:
$$ r^2 + h^2 = l^2 $$After substituting $r = 11\, \text{cm}$ and $h = 15\, \text{cm}$ we have:
$$ \left( 11\, \text{cm} \right)^{2} + \left( 15\, \text{cm} \right)^{2} = l^2 $$ $$ 121\, \text{cm}^2 + 225\, \text{cm}^2 = l^2 $$ $$ l^2 = 346\, \text{cm}^2 $$ $$ l = \sqrt{ 346\, \text{cm}^2 } $$$$ l = \sqrt{ 346 }\, \text{cm} $$