C. 13
In the circle, angle F is an inscribed angle. The arc measure of an inscribed angle is the inscribed angle times 2. So you set up the equation as;
(6x-8)2=10x+10
Distribute
12x-16=10x+10
Add 16 on both sides
12x=10x+26
Subtract 10x from both sides
2x=26
Divide 2 from both sides
X=13
