Two lines are perpendicular.  One line has a direction vector of $\begin{pmatrix} 3 \\ -7 \end{pmatrix}.$  The other line has a direction vector of $\begin{pmatrix} a \\ 2 \end{pmatrix}.$  Find $a.$
Since the two lines are perpendicular, their direction vectors are orthogonal.  This means that the dot product of the direction vectors is 0:
\[\begin{pmatrix} 3 \\ -7 \end{pmatrix} \cdot \begin{pmatrix} a \\ 2 \end{pmatrix} = 0.\]Then $3a - 14 = 0,$ so $a = \boxed{\frac{14}{3}}.$