To find side $ a $ use Pythagorean Theorem:

$$ h^2 + \frac{ a^2 }{ 4 } = s^2 $$

After substituting $h = 16.8\, \text{cm}$ and $s = 19.3\, \text{cm}$ we have:

$$ \left( 16.8\, \text{cm} \right)^{2} + \frac{ a^2 }{ 4 } = \left( 19.3\, \text{cm} \right)^{2} $$ $$ \frac{ a^2 }{ 4 } = \left( 19.3\, \text{cm} \right)^{2} - \left( 16.8\, \text{cm} \right)^{2} $$ $$ \frac{ a^2 }{ 4 } = 372.49\, \text{cm}^2 - 282.24\, \text{cm}^2 $$ $$ a^2 = 90.25\, \text{cm}^2 \cdot 4 $$ $$ a^2 = 361\, \text{cm}^2 $$ $$ a = \sqrt{ 361\, \text{cm}^2 } $$$$ a = 19\, \text{cm} $$

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