\[\left( 2\frac{1}{4}a + 1\frac{2}{3}b \right)^{2} =\]
\[= \left( \frac{9}{4}a + \frac{5}{3}b \right)^{2} =\]
\[= \frac{81}{16}a² + \frac{15}{2}ab + \frac{25}{9}b²\]