
To find long base $ a $ use formula:
$$ m = \frac{ a + b } { 2 } $$After substituting $m = 76\, \text{cm}$ and $b = 61\, \text{cm}$ we have:
$$ 76\, \text{cm} = \frac{ a + 61\, \text{cm} } { 2 } $$ $$ 76\, \text{cm} \cdot 2 = a + 61\, \text{cm} $$ $$ a + 61\, \text{cm} = 152\, \text{cm} $$ $$ a = 152\, \text{cm} - 61\, \text{cm} $$ $$ a = 91\, \text{cm} $$