if you want to calculate 3/5 + 5/9, you have to bring them to a common denominator; you do it by multiplying the nominator and denominator of both by the other denominator:
[tex] \frac{3}{5} + \frac{5}{9} = \frac{3*9}{5*9} + \frac{5*5}{9*5}= \frac{27+25}{5*9} = \frac{52}{45} =1 \frac{7}{45} [/tex]
[tex] \frac{12}{21} - \frac{14}{21} =-1 \frac{2}{21} [/tex]
