Bernoulli's Equation and pressure differences

[OnAHyperbola]

Homework Statement

Problem in attached image

Homework Equations

$$P_1+\frac{\rho v_1^2}{2}=P_2+\frac{\rho v_2^2}{2}$$

The Attempt at a Solution

I understand everything in the solution except why $$P_A-P_B=h(\rho_{Hg}-\rho)g$$ Why do we have to subtract the density of water from that of mercury instead of just taking the density of mercury??

Any help is appreciated.

 

[TSny]

Consider starting at point B and moving down through the U-tube all the way around until you come up the left side of the U-tube to point A. Keep track of all the pressure changes.