First calculate the diameter of circle.
[tex]d=8+4=12[/tex]
Now we know that diameter is equal for any two points on the arc of a circle lying opposite to each other.
[tex]12=x+4[/tex]
Now just simply solve for x.
[tex]
12=x+4 \\
12-4=x+4-4 \\
\boxed{x=8} \\
[/tex]
Hope this helps.
r3t40
