The picture is difficult to read, but it looks like
∠ADB = 2x²
∠ACB = 10x
These two angles have equal measure, so you have
2x² = 10x
2x(x -5) = 0 . . . . . subtract 10x and factor
x = 0, or x = 5
The measure of arc AB is twice the measure of either inscribed angle, so would be
arc AB = 2×10x
= 2×10×5
= 100 . . . . . degrees
Arc AB is 100°.
