A complex number is represented as A+iB where A and B are real number and i= √-1

Generally, complex number is denoted by z.

Argument fo a complex number can be calculated as

                      tanθ = B/A

                  => θ = tan^-1 (B/A)

                  =>  arg(z) =  tan^-1 (B/A)

Example: Let a complex number is z = 4 +3i

the argument of this complex number is 

                 tanθ = 3/4

            => θ = tan^-1 (3/4)

            =>  arg(z) = tan^-1 (3/4)