First we must change 12 dm to cm. 12 dm x 10 = 120 cm
Next we divide 120 cm by 4.3 cm = 27.907 radians which is the measure of the central angle.

If you have the arc length and the radius divide the former by the latter and the result is the
measure of the central angle in radians.

Remember that the arc lenght and radius must be in the same measurement.
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