6. a) -1/3 + (1/2); б) 7/15 - 13/15.

а) \(-\frac{1}{3} + \frac{1}{2} = -\frac{2}{6} + \frac{3}{6} = \frac{1}{6}\). б) \(\frac{7}{15} - \frac{13}{15} = \frac{7-13}{15} = \frac{-6}{15} = -\frac{2}{5}\).