Find the arc measure of ab. the measure of arc ab = degrees

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°.