The winding problem of Spiral Arms

The winding problem describes why spirals cannot be just material arms, i.e. stars that are lined up in that particular fashion. Consider a simple model of a material arm. Suppose that a group of stars start out lined up along the radial direction from a radius of 6 kpc to 10 kpc. Assume that the circular velocity of the galaxy is a constant at all radii, with a value of 200 km/s. Write an equation for the angular velocity of the stars as a function of radius. Your units should be degrees/million years (you can assume that 1 km/s = 1pc/Myr, where Myr is megayear = 1 million years).