Bonjour,
[tex]a = - \frac{2}{21} \: \: et \: \: b = \frac{5}{ - 7} [/tex]
[tex]a + b[/tex]
[tex] = - \frac{2}{21} + \frac{5}{ - 7} [/tex]
[tex] = - \frac{2}{21} - \frac{5}{7} [/tex]
[tex] = - \frac{2}{21} - \frac{5 \times 3}{7 \times 3} [/tex]
[tex] = - \frac{2}{21} - \frac{15}{21} [/tex]
[tex] = - \frac{17}{21} [/tex]