We must find Least Common Denominator (LCD).
List the multiples of 7: 7; 14; 21; 28; ...
List the multiples of 3: 3; 6; 9; 12; 15; 18; 21; 24; 27; ...
[tex]-\dfrac{4}{7}=-\dfrac{4\cdot3}{7\cdot3}=-\dfrac{12}{21}\\\\-\dfrac{4}{3}=-\dfrac{4\cdot7}{3\cdot7}=-\dfrac{28}{21}\\\\therefore\\\\-\dfrac{4}{7}+\left(-\dfrac{4}{3}\right)=-\dfrac{12}{21}+\left(-\dfrac{28}{21}\right)=-\dfrac{40}{21}=-1\dfrac{19}{21}[/tex]
