To find slant height $ s $ use Pythagorean Theorem:
$$ h^2 + \frac{ a^2 }{ 4 } = s^2 $$After substituting $ h = 6 $ and $ a = 6 $ we have:
$$ 6^2 + \frac{ 6^2 }{ 4 }= s^2 $$ $$ 36 + \frac{ 36 }{ 4 }= s^2 $$ $$ 36 + 9 = s^2 $$ $$ s^2 = 45 $$ $$ s = \sqrt{ 45 } $$$$ s = 3 \sqrt{ 5 } $$