To find height $ h $ use formula:
$$ A = a \cdot h $$After substituting $A = 30\, \text{cm}$ and $a = 12\, \text{cm}$ we have:
$$ 30\, \text{cm} = 12\, \text{cm} \cdot h $$$$ h = \dfrac{ 30\, \text{cm} }{ 12\, \text{cm} } $$$$ h = \frac{ 5 }{ 2 } $$