Answer: x = 11
To start, we know that a whole circle equals 360 degrees. If we subtract the 2 arcs we know, 85 and 31, we have 244 degrees left for the 2 congruent arcs. Dividing that by 2 gives us 122 for each arc.
Now, write the equation: 122 = 13x - 21
Solve this, to find the x = 11.
