#### Answer

$\dfrac{4a-4}{7}$

#### Work Step by Step

When adding two fractions with common denominators, we add the numerators and copy the denominator. Therefore, the given expression, $
\dfrac{a}{7}+\dfrac{3a-4}{7}
,$ simplifies to
\begin{array}{l}\require{cancel}
\dfrac{a+3a-4}{7}
\\\\=
\dfrac{4a-4}{7}
.\end{array}