\[\left( 3x^{5} - \frac{1}{3}x^{6}y \right)^{2} =\]
\[= 9x^{10} - 2 \cdot 3 \cdot \frac{1}{3}x^{5}x^{6}y + \frac{1}{9}x^{12}y^{2} =\]
\[= 9x^{10} - 2x^{11}y + \frac{1}{9}x^{12}y^{2}.\]