A spring is suspended freely. When a mass of 20 g is attached to the spring, it has a length of 12 cm. When a mass of 50 g is attached to the spring, it has a length of 15 cm. The graph below shows how the length, y em, of the spring, varies with the mass, xg, attached to it. (50, 15) (20, 12) (i) Find an expression for y in terms of x. (ii) State what the value of the y-intercept represents.