
To find long base $ a $ use formula:
$$ m = \frac{ a + b } { 2 } $$After substituting $m = 80\, \text{cm}$ and $b = 64\, \text{cm}$ we have:
$$ 80\, \text{cm} = \frac{ a + 64\, \text{cm} } { 2 } $$ $$ 80\, \text{cm} \cdot 2 = a + 64\, \text{cm} $$ $$ a + 64\, \text{cm} = 160\, \text{cm} $$ $$ a = 160\, \text{cm} - 64\, \text{cm} $$ $$ a = 96\, \text{cm} $$