Radian measure is given by dividing the length of an arc by the radius of a given circle. Such that an arc that subtends an angle of 1 radian to the center of a circle is equivalent to the radius of the circle. Therefore; θ = s/r where θ is the angle subtended in radians, s is the length of the arc and r is the radius of the circle. Hence; r = s/θ = 12π ÷ (4π/7) = 12 × (7/4) = 21 inches
