We know that F_gravity = G * M_star * m_planet / r² is the force that keeps planets moving in their (essentially circular) orbits around a star, where G is the gravitational constant: 6.7 × 10⁻¹¹ m³ s⁻² (kg)⁻². For any object moving in a circle, the magnitude of the net force operating upon it is equal to a known function of its mass, speed, and the radius of the circle in which it moves. Since F_gravity is the force responsible for the circular motion of the planet, its magnitude can be set equal to this function. Do so, and then cancel the variables that can be cancelled. (Feel free to review the section on circular motion if necessary!)