A rectangular channel $3\mathrm{.}0$ m wide has a longitudinal slope of $150$ mm$/$km and Manning's coefficient $0\mathrm{.}02.$ When the discharge in the channel is $0\mathrm{.}85$ m$\text{'}$s$!,$ determine the slope of the water surface in the channel relative to the horizontal, i$.$e$.($dyl dx $-$ So$)$ at a location where the depth of flow is $0\mathrm{.}75$ m$.(3)$