[tex]\displaystyle\\
\frac{2}{7} = \frac{6}{21} \\\\\\
a)~~~\frac{2}{7-2}=\frac{6}{21-6}~~~\Longleftrightarrow~~~\frac{2}{5} = \frac{6}{15}\\\\\\
b)~~~\frac{2:2}{7} = \frac{6:2}{21} ~~~\Longleftrightarrow~~~ \frac{1}{7} = \frac{3}{21}\\\\\\
c)~~~ \frac{2}{7:7} = \frac{6}{21:7}~~~\Longleftrightarrow~~~ \frac{2}{1} = \frac{6}{3}[/tex]