To find slant height $ s $ use Pythagorean Theorem:
$$ h^2 + \frac{ a^2 }{ 4 } = s^2 $$After substituting $ h = 10 $ and $ a = 8 $ we have:
$$ 10^2 + \frac{ 8^2 }{ 4 }= s^2 $$ $$ 100 + \frac{ 64 }{ 4 }= s^2 $$ $$ 100 + 16 = s^2 $$ $$ s^2 = 116 $$ $$ s = \sqrt{ 116 } $$$$ s = 2 \sqrt{ 29 } $$