At first we should know that :
Foe any parallelogram, the diagonals bisects each other
so, for the given figure :
[tex]6x-8=4x[/tex]Solve for x :
[tex]\begin{gathered} 6x-4x=8 \\ 2x=8 \\ \\ x=\frac{8}{2}=4 \end{gathered}[/tex]so, the answer is : x = 4