To find side $ a $ use Pythagorean Theorem:
$$ a^2 + b^2 = c^2 $$After substituting $ b = 10 $ and $ c = 20 $ we have:
$$ a ^ {\,2} + 10^2 = 20^2 $$ $$ a ^ {\,2} = 20^2 - 10^2 $$ $$ a ^ {\,2} = 400 - 100 $$ $$ a ^ {\,2} = 300 $$ $$ a = \sqrt{ 300 } $$$$ a = 10 \sqrt{ 3 } $$