[tex] \frac{8}{7} - \frac{1}{3} = \frac{17}{21}[/tex]
because in order to solve you need to get the common denominator, which in this case is 21...
[tex] \frac{3}{3} ( \frac{8}{7} ) = \frac{24}{21} [/tex]
[tex] \frac{7}{7} ( \frac{1}{3} ) = \frac{7}{21} [/tex]
and now that you have a common denominator you can complete the operation...
[tex] \frac{24}{21} - \frac{7}{21} = \frac{17}{21} [/tex]
