5/21+4/9;
-5/21-4/9;
5/21-4/9;
-5/21+4/9

$frac{5}{21} + frac{4}{9} = frac{5*3}{21*3} + frac{4*7}{9*7} = frac{15+28}{63} = frac{43}{63}$

$- frac{5}{21}- frac{4}{9} =- frac{5*3}{21*3} - frac{4*7}{9*7} =- frac{43}{63}$

$frac{5}{21}- frac{4}{9} = frac{5*3}{21*3}- frac{4*7}{9*7} = frac{15-28}{63} = frac{-13}{63}$

$- frac{5}{21} + frac{4}{9} = frac{4}{9} - frac{5}{21} = frac{4*7}{9*7} - frac{5*3}{21*3} = frac{28-15}{63} = frac{13}{63}$