Answer:
∠72°.
Step-by-step explanation:
If a circle has a circumference of 20 and an arc with a length of 4, we can simply set up a ratio to determine the angle of the arc in degrees:

Cross multiply:

Divide both sides by '20':
x = 72°.
Therefore, the measure of the central angle of the arc is ∠72°.
Answer:
The solution is in the attached file below
x^2 - 81 = 0. Add 81 to each side.
x^2 = 81. Take the square root of each side.
x = 9, -9