
To find long base $ a $ use formula:
$$ m = \frac{ a + b } { 2 } $$After substituting $m = 20\, \text{cm}$ and $b = 14\, \text{cm}$ we have:
$$ 20\, \text{cm} = \frac{ a + 14\, \text{cm} } { 2 } $$ $$ 20\, \text{cm} \cdot 2 = a + 14\, \text{cm} $$ $$ a + 14\, \text{cm} = 40\, \text{cm} $$ $$ a = 40\, \text{cm} - 14\, \text{cm} $$ $$ a = 26\, \text{cm} $$