
To find long base $ a $ use formula:
$$ m = \frac{ a + b } { 2 } $$After substituting $m = 10.8\, \text{cm}$ and $b = 9.7\, \text{cm}$ we have:
$$ 10.8\, \text{cm} = \frac{ a + 9.7\, \text{cm} } { 2 } $$ $$ 10.8\, \text{cm} \cdot 2 = a + 9.7\, \text{cm} $$ $$ a + 9.7\, \text{cm} = 21.6\, \text{cm} $$ $$ a = 21.6\, \text{cm} - 9.7\, \text{cm} $$ $$ a = 11.9\, \text{cm} $$