
To find long base $ a $ use formula:
$$ m = \frac{ a + b } { 2 } $$After substituting $m = 25\, \text{cm}$ and $b = 18\, \text{cm}$ we have:
$$ 25\, \text{cm} = \frac{ a + 18\, \text{cm} } { 2 } $$ $$ 25\, \text{cm} \cdot 2 = a + 18\, \text{cm} $$ $$ a + 18\, \text{cm} = 50\, \text{cm} $$ $$ a = 50\, \text{cm} - 18\, \text{cm} $$ $$ a = 32\, \text{cm} $$