
To find short base $ b $ use formula:
$$ m = \frac{ a + b } { 2 } $$After substituting $m = 49\, \text{cm}$ and $a = 64\, \text{cm}$ we have:
$$ 49\, \text{cm} = \frac{ 64\, \text{cm} + b } { 2 } $$ $$ 49\, \text{cm} \cdot 2 = 64\, \text{cm} + b $$ $$ 64\, \text{cm} + b = 98\, \text{cm} $$ $$ b = 64\, \text{cm} + 98\, \text{cm} $$ $$ b = 162\, \text{cm} $$