
To find short base $ b $ use formula:
$$ m = \frac{ a + b } { 2 } $$After substituting $m = 30\, \text{cm}$ and $a = 40\, \text{cm}$ we have:
$$ 30\, \text{cm} = \frac{ 40\, \text{cm} + b } { 2 } $$ $$ 30\, \text{cm} \cdot 2 = 40\, \text{cm} + b $$ $$ 40\, \text{cm} + b = 60\, \text{cm} $$ $$ b = 40\, \text{cm} + 60\, \text{cm} $$ $$ b = 100\, \text{cm} $$