\[\left( 2\frac{1}{7} \right)^{2} - \left( 2\frac{1}{7} \right)^{2} =\]
\[= \left( 2\frac{1}{7} - 2\frac{1}{7} \right)\left( 2\frac{1}{7} + 2\frac{1}{7} \right) =\]
\[= 0 \cdot 4\frac{2}{7} = 0.\]