Preliminary Work
Identify design variables
form constraint equation in terms of design variables
form objective function in terms of design variables

Using Excel’s Solver
Input design variables
q
Input objective function

Design variable-C2
Objective function - E2

Pull down TOOL menu
select Solver

Target Cell=objective function (E2)
By Changing (design variables)C2
Choose Solve to start Solver

Solution: 
optimal velocity occurs when q = 79.3o
choose ok