Verified answer

3)

[tex] \frac{17}{21} - \frac{1}{2} = \frac{34 - 21}{42} = \frac{13}{42} [/tex]

4)

[tex] \frac{9}{10} - \frac{7}{9} = \frac{81 - 70}{90} = \frac{11}{90} [/tex]

5)

[tex] \frac{14}{21} - \frac{3}{5} = \frac{70 - 63}{105} = \frac{1}{15} [/tex]