$2\mathrm{.}12\mathrm{.}9$ The U$-$tube manometer in Fig. P$2\mathrm{.}12\mathrm{.}9$ is used to measure the

aeceleration of the cart on which it sits. Develop an expression for

the acceleration of the cart in terms of the liquid height $h,$ the liquid

density $,$ the local acceleration of gravity $g,$ and the length $l.$