To find side $ a $ use formula:
$$ d_1^2 + d_2^2 = 4 \cdot a^2 $$After substituting $ d_1 = 15 $ and $ d_2 = 6 $ we have:
$$ 15^2 + 6^2 = 4 \cdot a^2 $$ $$ 225 + 36 = 4 \cdot a^2 $$ $$ 4 \cdot a^2 = 261 $$ $$ a^2 = \frac{ 261 }{ 4 } $$ $$ a^2 = \frac{ 261 }{ 4 } $$ $$ a = \sqrt{ \frac{ 261 }{ 4 } } $$$$ a = \frac{ 3 \sqrt{ 29}}{ 2 } $$