An arc of a circle of radius 12 cm subtends an angle of 30° at the centre. The length of the arc is:

The formula arc = $r$·θ needs θ in radians: 30° = π/6, so the arc is 12 × π/6 = 2π ≈ 6.28 cm. Feeding the degree number straight into $r$·θ gives the absurd $12\times 30=360$ cm; 12π ${\text{cm}}^2$ is the area of the sector, not a length; and π/6 is the angle itself.