To find slant height $ s $ use Pythagorean Theorem:
$$ h^2 + \frac{ a^2 }{ 4 } = s^2 $$After substituting $ h = 10 $ and $ a = 15 $ we have:
$$ 10^2 + \frac{ 15^2 }{ 4 }= s^2 $$ $$ 100 + \frac{ 225 }{ 4 }= s^2 $$ $$ 100 + \frac{ 225 }{ 4 } = s^2 $$ $$ s^2 = \frac{ 625 }{ 4 } $$ $$ s = \sqrt{ \frac{ 625 }{ 4 } } $$$$ s = \frac{ 25 }{ 2 } $$