STEP 1: find side $ a $

To find side $ a $ use formula:

After substituting $B = 20\, \text{cm}$ we have:

$$ B = a^2 $$ $$ a^2 = 20\, \text{cm} $$ $$ a = \sqrt{ 20\, \text{cm} } $$ $$ a = 2 \sqrt{ 5 } $$

STEP 2: find height $ h $

To find height $ h $ use formula:

$$ V = \dfrac{ a ^{ 2 } \cdot h }{ 3 } $$

After substituting $V = 140\, \text{cm}$ and $a = 2 \sqrt{ 5 }\, \text{cm}^0$ we have:

$$ 140\, \text{cm} = \dfrac{ 2 \sqrt{ 5 } ^{ 2 } \cdot h }{ 3 } $$$$ 140\, \text{cm} \cdot 3 = 2 \sqrt{ 5 } ^{ 2 } \cdot h $$$$ 420\, \text{cm} = 20 \cdot h $$$$ h = \dfrac{ 420\, \text{cm} }{ 20 } $$$$ h = 21\, \text{cm} $$

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