
To find long base $ a $ use formula:
$$ m = \frac{ a + b } { 2 } $$After substituting $m = 22\, \text{cm}$ and $b = 13\, \text{cm}$ we have:
$$ 22\, \text{cm} = \frac{ a + 13\, \text{cm} } { 2 } $$ $$ 22\, \text{cm} \cdot 2 = a + 13\, \text{cm} $$ $$ a + 13\, \text{cm} = 44\, \text{cm} $$ $$ a = 44\, \text{cm} - 13\, \text{cm} $$ $$ a = 31\, \text{cm} $$