An operation is defined on the set of positive real numbers as
$$a \bullet b = \frac { a \cdot b } { a + b }$$
Given that
$$\frac { 1 } { 2 } \bullet \frac { 3 } { 4 } = 3 \bullet \frac { 1 } { x }$$
what is $x$?\\
A) $\frac { 3 } { 2 }$\\
B) $\frac { 9 } { 4 }$\\
C) 2\\
D) 3\\
E) 5