Accelerating U-tube

A U-tube with a very small diameter, vertical sections of height h and horizontal section of length 2h is filled with mercury of density ρ such that the mercury only occupies the horizontal section. The U-tube is capped tightly at both ends and then accelerated to the right such that the mercury column occupies a length of $\frac { 5h }{ 3 }$ inside the horizontal section. Atmospheric pressure is ${ P }_{ 0 }$ . If the acceleration of the tube can be expressed as $\frac { { AP }_{ 0 }+B\rho gh }{ C\rho h }$ , where A, B and C are positive integers, find the values of C-(A+B).