To find side $ a $ use Law of Cosines:

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

After substituting $b = 15\, \text{cm}$, $c = 120\, \text{cm}$ and $\alpha = 58^o$ we have:

$$ a^2 = 15^2 + 120^2 - 2 \cdot 15 \cdot 120 \cdot \cos( 58^o ) $$ $$ a^2 = 225\, \text{cm}^2 + 14400\, \text{cm}^2 - 2 \cdot 15 \cdot 120 \cdot \cos( 58^o ) $$ $$ a^2 = 14625\, \text{cm}^2 - 2 \cdot 1800\, \text{cm}^2 \cdot \cos( 58^o ) $$ $$ a^2 = 14625\, \text{cm}^2 - 3600\, \text{cm}^2 \cdot 0.5299 $$ $$ a^2 = 14625\, \text{cm}^2 - 1907.7094\, \text{cm}^2 $$ $$ a^2 = 12717.2906\, \text{cm}^2 $$ $$ a = \sqrt{ 12717.2906\, \text{cm}^2 } $$$$ a \approx 112.771\, \text{cm} $$

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