Since \( FK \) is the perpendicular bisector of \( GJ \), it divides \( GJ \) into two equal segments. Therefore, we can set the lengths of segments \( GF \) and \( JF \) equal to each other:
\[ GF = JF \]
Given that:
\[ GF = 6 + x \] \[ JF = 3x - 5 \]
Setting the equations equal gives us:
\[ 6 + x = 3x - 5 \]
Now, we'll solve for \( x \):
- Subtract \( x \) from both sides:
\[ 6 = 2x - 5 \]
- Add \( 5 \) to both sides:
\[ 6 + 5 = 2x \] \[ 11 = 2x \]
- Divide both sides by \( 2 \):
\[ x = \frac{11}{2} = 5.5 \]
Thus, the value of \( x \) is
\[ \boxed{5.5} \]