\[\frac{81}{16}a^{6} - 9a^{3}b^{2} + 4b^{4} =\]
\[= \left( \frac{9}{4}a^{3} - 2b^{2} \right)^{2} =\]
\[= \left( 2\frac{1}{4}a^{3} - 2b^{2} \right)^{2}\]