The volume \( V \) of a pyramid is given by the formula:
\[ V = \frac{1}{3} \times \text{Base Area} \times \text{Height} \]
For a rectangular pyramid:
\[ \text{Base Area} = \text{Length} \times \text{Width} = 7 , \text{cm} \times 9 , \text{cm} = 63 , \text{cm}^2 \]
Substituting into the volume formula:
\[ 231 = \frac{1}{3} \times 63 \times h \]
Multiplying both sides by 3:
\[ 693 = 63h \]
Solving for \( h \):
\[ h = \frac{693}{63} = 11 , \text{cm} \]
Thus, the height of the pyramid is 11 cm.