To find height $ h $ use formula:
$$ A = a \cdot h $$After substituting $A = 87\, \text{cm}$ and $a = 10\, \text{cm}$ we have:
$$ 87\, \text{cm} = 10\, \text{cm} \cdot h $$$$ h = \dfrac{ 87\, \text{cm} }{ 10\, \text{cm} } $$$$ h = \frac{ 87 }{ 10 } $$