
To find short base $ b $ use formula:
$$ m = \frac{ a + b } { 2 } $$After substituting $m = 39\, \text{cm}$ and $a = 47\, \text{cm}$ we have:
$$ 39\, \text{cm} = \frac{ 47\, \text{cm} + b } { 2 } $$ $$ 39\, \text{cm} \cdot 2 = 47\, \text{cm} + b $$ $$ 47\, \text{cm} + b = 78\, \text{cm} $$ $$ b = 47\, \text{cm} + 78\, \text{cm} $$ $$ b = 125\, \text{cm} $$