# Lever rule confusion

At point a3, I know the ratio of the liquid phase to the solid phase, but I don't get the highlighted statement which says b3 gives the composition. I thought a3 already gave the composition? Do I need to apply another lever rule where b3 is the center? If so, where would the endpoints be?

The statement refers to the composition of the liquid phase, which is simply the x-value of $b_3$.