Find the measure of ED. From the theorem: an angle formed by a chord and a tangent that intersect on a circle is half the measure of the intercepted arc.
So, the answer clearly is 4x - 8 = 80/2 (80 is the angle).
To solve it take both fraction to the same denominator.
(8x - 16)/2 = 80/2
Multiply both fractions for 2 to delete denominators.
8x - 16 = 80
Move - 16 to the right of equal with changed sign.
8x = 80 + 16
8x = 96
8x/8 = 96/8
x = 12
Answer C