You must find a common denominator before you can combine these terms.

A common denominator is often a multiple of both denominators of the fractions that are to be combined.
A good one to use for this expression would be $30$.

$\frac{3}{7}+$  $\frac{6}{14}$

$\frac{6}{14}-\frac{7}{14}=-\frac{1}{14}$

$-\frac{1}{14}$

Refer to part (a).