simple steps without explanation

Question 3 The average volumetric flow through an open rectangular duct can be estimated using the following empirical equation: (B.) Q VS (B + 2H)3 NI n Where: Q = flow rate in m/s B = width in m H = depth in m S = slope (dimensionless) n = roughness coefficient (dimensionless) a) What are the units of the constant 2? b) Derive an equation for Q to be in ft/s and both B and H in ft