If a variable line drawn through the intersection of the lines $\frac { x } { 3 } + \frac { y } { 4 } = 1$ and $\frac { x } { 4 } + \frac { y } { 3 } = 1$, meets the coordinate axes at $A$ and $B$, $( A \neq B )$, then the locus of the midpoint of $AB$ is:\\
(1) $7 x y = 6 ( x + y )$\\
(2) $4 ( x + y ) ^ { 2 } - 28 ( x + y ) + 49 = 0$\\
(3) $6 x y = 7 ( x + y )$\\
(4) $14 ( x + y ) ^ { 2 } - 97 ( x + y ) + 168 = 0$