Question $3$ Water Engineering

A trapezoidal channel with equal side slopes is lined with asphalt $(n=0\mathrm{.}016)$ and is used to transport water at a flow rate of $120\frac{m^3}{s}.$ Determine the slope of the channel if the channel has a flow depth of $2\mathrm{.}2$ m $,$ a top width of $12$ m $,$ and a bottom width of $6$ m $.$