Substraction of fractions:
[tex]\frac{a}{b}-\frac{d}{e}=\frac{a\cdot e-b\cdot d}{b\cdot e}[/tex]For the given substraction:
[tex]\frac{6}{7}-\frac{3}{4}=\frac{6\cdot4-7\cdot3}{7\cdot4}=\frac{24-21}{28}=\frac{3}{28}[/tex]Then the result of the given substraction is 3/28