For the complex number z is equal to negative the fraction with numerator square root of 21 and denominator 2 minus the fraction with numerator square root of 7 and denominator 2 i , what is arg z ?

The real part is −√21/2 and the imaginary part is −√7/2, so z lies in Quadrant III. Its reference angle satisfies tan θ = (√7/2)/(√21/2) = 1/√3, giving 30°; therefore arg z = 180° + 30° = 210°, choice B.

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