To find side $ a $ use Law of Cosines:
$$ a^2 = b^2 + c^2 - 2 \cdot b \cdot c \cdot \cos( \alpha ) $$After substituting $b = 3$, $c = 9$ and $\alpha = 108^o$ we have:
$$ a^2 = 3^2 + 9^2 - 2 \cdot 3 \cdot 9 \cdot \cos( 108^o ) $$ $$ a^2 = 9 + 81 - 2 \cdot 3 \cdot 9 \cdot \cos( 108^o ) $$ $$ a^2 = 90 - 2 \cdot 27 \cdot \cos( 108^o ) $$ $$ a^2 = 90 - 54 \cdot \left(-0.309\right) $$ $$ a^2 = 90 - \left(-16.6869\right) $$ $$ a^2 = 106.6869 $$ $$ a = \sqrt{ 106.6869 } $$$$ a \approx 10.3289 $$