Answer
$D=43.5\;km$
Work Step by Step
The pressures at points $a$ and $b$ are equal
$P_a=P_b$
$\rho_c g(H+T+D)+\rho_m g(y-D)=\rho_c g T+\rho_m g y$
or, $\rho_c D-\rho_m D=\rho_c T-\rho_c (H+T)$
or, $D=\frac{\rho_c T-\rho_c (H+T)}{\rho_c -\rho_m }$
or, $D=\frac{\rho_c H}{\rho_m -\rho_c }$
Substituting the given values,
$D=\frac{2.9\times 6}{3.3 -2.9}\;km$
or, $\boxed{D=43.5\;km}$
