To find side $ a $ use Law of Cosines:

$$ a^2 = b^2 + c^2 - 2 \cdot b \cdot c \cdot \cos( \alpha ) $$

After substituting $b = 6\, \text{cm}$, $c = 4\, \text{cm}$ and $\alpha = 85^o$ we have:

$$ a^2 = 6^2 + 4^2 - 2 \cdot 6 \cdot 4 \cdot \cos( 85^o ) $$ $$ a^2 = 36\, \text{cm}^2 + 16\, \text{cm}^2 - 2 \cdot 6 \cdot 4 \cdot \cos( 85^o ) $$ $$ a^2 = 52\, \text{cm}^2 - 2 \cdot 24\, \text{cm}^2 \cdot \cos( 85^o ) $$ $$ a^2 = 52\, \text{cm}^2 - 48\, \text{cm}^2 \cdot 0.0872 $$ $$ a^2 = 52\, \text{cm}^2 - 4.1835\, \text{cm}^2 $$ $$ a^2 = 47.8165\, \text{cm}^2 $$ $$ a = \sqrt{ 47.8165\, \text{cm}^2 } $$$$ a \approx 6.9149\, \text{cm} $$

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