A particle is traveling with a constant velocity v along a line that is a distance b

Question:

A particle is traveling with a constant velocity v along a line that is a distance b from the origin O (Figure). Let dA be the area swept out by the position vector from O to the particle in time dt. Show that dA/dt is constant in time and equal to 1/2L/m, where L is the angular momentum of the particle about theorigin.