To find side $ a $ use Pythagorean Theorem:
$$ h^2 + \frac{ a^2 }{ 4 } = s^2 $$After substituting $ h = 65 $ and $ s = 66 $ we have:
$$ 65^2 + \frac{ a^2 }{ 4 } = 66^2 $$ $$ \frac{ a^2 }{ 4 } = 66^2 - 65^2 $$ $$ \frac{ a^2 }{ 4 } = 4356 - 4225 $$ $$ a^2 = 131 \cdot 4 $$ $$ a^2 = 524 $$ $$ a = \sqrt{ 524 } $$$$ a = 2 \sqrt{ 131 } $$