To find side $ a $ use Pythagorean Theorem:

$$ a^2 + b^2 = c^2 $$

After substituting $b = 4\, \text{cm}$ and $c = 5\, \text{cm}$ we have:

$$ a ^ {\,2} + \left( 4\, \text{cm} \right)^{2} = \left( 5\, \text{cm} \right)^{2} $$ $$ a ^ {\,2} = \left( 5\, \text{cm} \right)^{2} - \left( 4\, \text{cm} \right)^{2} $$ $$ a ^ {\,2} = 25\, \text{cm}^2 - 16\, \text{cm}^2 $$ $$ a ^ {\,2} = 9\, \text{cm}^2 $$ $$ a = \sqrt{ 9\, \text{cm}^2 } $$$$ a = 3\, \text{cm} $$

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